Is it possible to have an external developer in Xcode just using exported cert+prov Is it possible to have an external developer in Xcode without being logged in with an account, but using a exported certificate and provisioning profile?
Is it possible to select an imported certificate and provisioning profile without signing in with the account in Xcode?

We do not want him to have access to our Apple developer account, but need him to use a specific provisioning profile, to be able to send out test versions.

How do you solve this?
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.