Impex is a very flexible system. I'm not sure if Jelsoft would have the resources available to offer you an import solution to your custom forum software, as they are more concerned with offering importing options to people who use prebuilt forum solutions.
However, there are people out there with experience with Impex, and who would be able to write you some code which sits on top of Impex, and would let you import as much of your data as possible