The rule for resolution of a Garrison action states: "Remove one Battle Unit from the acting Legion's Holding Box. Place a Garrison counter on the map in the Tribal Box." So can an Auxiliary that is in that Legion's Holding Box be used as the Battle Unit that gets turned into a Garrison counter?
No, only a purple Legionary Battle Unit can be used to Garrison.
Darn-- I always think I have a game that's errata-free, and then something always pops up!
Thanks for the question, and for spotting this.