Use Adjuncts, renumber them to something within your dial plan that makes sense.

Assign them an "authorization code", so you can program their forwarding number from another (real) system phone. (Hint: make the authorization code be the same as the extension number. i.e., if the adjunct is extension 199, make the authorization code 199 as well.)

Be sure to allow them trunk-to-trunk and remote call forwarding.

To set the remote number, go to a keyset, go off hook on an SA button, and dial:

#80-(authorization code)-#-*33-9-(remote telephone number)-#