They are zips, and once opened they can be extracted and opened in MilkShape. Once you get there you'll have to rearrange the parts (if they're separated, like Quake 3 files) then re-texture them with the textures in the .pk3 file. Then you'd have to re-weight them to UTXX skeletons and export them. I don't think it'd be difficult if you know what you're doing (weighting is undoubtedly the most difficult part), and I doubt it'd be even too time consuming. The main thing is if you're doing a port, get the original artist's permission first, or else don't release it.