I had an expert contract The Blues on an expedition.  It said it could be treated at the healing retreat, but when I went there to send him, the retreat said I had '0 qualified' options to send. 

I stopped playing that museum for a while so I didn't lose him (since he maxed at 25% happiness and was always on the verge of quitting) but I did go back some time later, and for whatever reason it did finally recognize him (it was at least a month+ gametime before they would finally accept him).

I have screenshots if you need them, just to confirm this actually happened.