/topic/10844/
Part of the message is indeed broken there and cannot be restored, but the general idea should be clear.
In short, you need to unpack
data.pak using any archiver (like for a ZIP file) and extract the textures in DDS format from it. DDS is a graphics format; use Paint.NET, Photoshop with a plugin, or something else.Then repack them into a ZIP archive while preserving the folder structure, and give it the extension .h5u. Place it in UserMODs.
Here's a guide on how to do more advanced things, like importing entire models:
/topic/14904/
Overall, pay attention to this section:
/forum/section/285/