Is it possible to have an external developer in Xcode just using exported cert+prov
If you don't want him to have limited, handing your cert is not the way to go.
Add him as a team member on iTunes Connect, adjust his permissions and remove him when the QA ends. 

To add a second user to your account:


https://itunesconnect.apple.com/WebObjects/iTunesConnect.woa/ra/ng/users_roles


Add the Apple ID for the person you want to be able to sign and don't let them upload.  This works for individual accounts as well as enterprise / corporate accounts.