A tech artist is generally expected to know how to moded and texture stuff. That's a key part of what makes them a tech artist instead of an engineer. Though they're unlikely to be as good at it as a non-tech artist.
There is lots of freelance tech art. I mostly work with smaller indie studios that don't have a tech artist. I think Paul Neale (http://paulneale.com/) is a great example of a freelance tech artist.