I'm not sure if it's relevant to the version you're looking to mod, but here's a result for a QuickBMS script. But before trying to go down that route from scratch, you could try doing a Google search for QuickBMS Tony Hawk Underground 2 or MultiEx Tony Hawk Underground 2. There are also mod sections on each respective forum, which may interest you. There are tutorial sections on each site where you can learn about reverse engineering file formats and creating scripts however, this is by no means an easy or simple thing. Those programs are MultiEx (from XeNTaX) and QuickBMS (from ZenHAX). There are two noteworthy communities dedicated to reversing file formats with video games: XeNTaX and ZenHAXīoth of them have their own respective applications used for creating scripts that unpack files (and sometimes, repack).
