It's possible, though I don't think scripts exist to do it manually. You'd basically need to move the data in the Mambo database into Geeklog's database transforming the data as possible. I don't know what Mambo's database schema looks like, but this could be anywhere from really easy to really hard.
If you don't think you can tackel the task yourself, it might be possible to find someone willing to create the necessary conversion scripts.
-Vinny