I've worked through this with the forum base you talk about in the past with Kentico v6. The client wanted to have all old posts and images imported from the old site to the new Kentico site. Unfortunately, in v6 or v7 (or v8 I believe) there is no import tool for forum posts. I started to write some code to do the import although the client decided it was too much effort.
Kentico uses post id's to help generate the URLs so its not a straightforward import process not to mention the mapping of the different systems.
Regarding the API use, you are off to a good start. You'd need the following:
1. All user ID's who have posted (assuming UserID is stored in Telligent)
2. Import users using Import Toolkit from Kentico (assuming license allows) and use existing UserID
3. Create forum group
4. Create forum posts using recurrsion to ensure the posts are linked from top to bottom thread. Note, There is a limitation of roughly 63 replies to a post(at least in v6 and v7) because of SQL Server Full-Text search limitation of 450 characters. This goes back to what I mentioned before about using the post ID's to generate a URL.
Hopefully this helped a little!