This depends on the host in question…
It is possible for users of InvisionFree (or any install of self-hosted IPB, including IPB 3.x) to transfer to JFH. The process is generally to register a new forum (if you don't already have a forum). Don't make any changes (that you want kept anyway) to your forum because they'll be overwritten when the database gets imported. Then contact Jcink (email@example.com) and provide him with the database.
Warning - Transferring from InvisionFree does cost money. They (InvisionFree) charge users for each copy of the database they made. To find out how much it will cost and to acquire the database from InvisionFree you will need to submit a support ticket to them asking about back ups. Costs will range depending on the size of your forum.
Much like IPB / InvisionFree, users of self-hosted MyBB or IcyBoards can also transfer over to JFH. You'll need a copy of your database. (You'll need to check with IcyBoards to see how to get that and be aware that they may charge a flat rate of 20$ to obtain a backup.)
Then speak to Jcink (and pass along the database).
Note - Due to the conversion process, users transferring from MyBB based software will have to reset their passwords (using the forgot password recovery page on that forum). This is because MyBB stores passwords differently than IPB does and the passwords are rendered unusable after conversion (thus requiring a reset).
vBulletin is also supported for transfer into JFH. Though you will need to speak to Jcink for more info about this one.
However if your database is from another host then it may not possible at this time for you to transfer your database into Jcink Forum.
The primary reason is that no converter for that host to Jcink Forum Hosting exists. Jcink does not have time to create all of the converters that would be needed to enable all other forum types to transfer to JFH. Given the relatively low number of certain forums (like SMF) that would consider making the jump in the first place, it just isn't worth that time when there are many other important projects in the works.
You can get a copy of your database (using these instructions), however you will have to speak to the technical support for the service you desire to transfer to. Jcink does not (and cannot) offer support for those services.
For a heads up, if you are considering a transfer it is highly likely that you will lose data in the conversion. Jcink Forums runs a very customized version of IPB with a lot of extra features. Data from these features will likely be lost in the conversion. (Examples of data that will likely be lost are: Store items, point values, portal set up, web pages, etc…) Data that should not be lost include posts, users, user groups and forums (permissions may have to be reset though).
WARNING - All risks associated with any conversions are undertaken at the user's own risk, JFH is not liable for any data loss that occurs and does not provide support for any issues that occur as a result.
Attempting to import a database made for one forum into another is known to cause issues. Data loss and fragmentation resulting in broken and irreparable links and generally is more trouble than it is worth. It is also policy to only allow a database taken from one board to be used on that same board.
Much like attempting to import a database from one forum to another, modifying a database is known to cause problems if edited improperly. (Wrong values entered, values that exceed limits, etc…) which can compromise the integrity of not only the database of that forum, but of other user's forums too. So to ensure integrity of your data, users will not be allowed to have any/all modified databases accepted.
Users cannot upload the database by themselves because enabling users to access the server side functions would entail too much risk for everyone.
Much like the reason why board url changes are not permitted (users get careless and their boards get lost), users who don't understand how MySQL or a database works could be a great danger to their forum. Furthermore, malicious users could use this feature to great success against board owners. (Erasing boards, filling them with spam or other ToS violating content, affecting other users through compromised databases, etc…) Even users who know how a database works could still end up causing harm to their database or other's databases by uploading modified databases.
Therefore, for the benefit of everyone using JFH, only Jcink can upload a database.
Attempting to merge databases isn't possible because:
- the sheer amount of conflict resolution would take a long time and would be extremely difficult. (Since each database would have duplicate post ids. Depending on forum size, upwards of several thousand posts, each with different content and timestamps.)
- links would be broken due to importing new posts and any conflict resolution.
- it could cause errors in the database resulting in lost data for that forum (especially if the structures of the two databases are different) and possibly others on the service if this database were uploaded.
So database merges aren't done (even if you want to perform the merger yourself, it still wouldn't be uploaded due to the above listed reasons and these reasons.)