Sorry, that should have been: mgrs/libmgrs/utm.c at main · hobuinc/mgrs · GitHub Convert_Geodetic_To_UTM()
XForms does support pretty much a full suite of mathematical and trig functions - see Form Operators and Functions - ODK Docs
So if you can find a suitable algorithm for performing a GPS lat+long to UTM conversion, it should be possible to implement a suitable set of calculations within your form to generate a UTM result. (eg I’ve done this for converting geopoint/geoshape/geotrace to KML). So yes, its quite likely possible to ‘do that inside odk or Kob’, it just may be a bit tricky. 
