The importer includes a clean framework for building custom importers. A new importer can be built by an experienced developer in as little as 2-3 days!
To get started with building an importer, take a look at one of the existing ones under the importers folder.
In most cases you just need to create a new file and fill out a set of array options that will tell the importer engine where the old forum data is located.
A typical mapping would look similar to the following: