I don't completely understand how you've got this set up, but if the Axxess is truly networked to 5000 you'll be able to reach any 5000 extension from Axxess. So if ext 1234 on 5000 is forwarded to 1234 on hosted system, dialing that ext on Axxess will still get you there. I'd say build all the SIP destinations as phantom ext on 5000, manually fwd each to SIP host (using trunk access codes and other needed digits). Then you can dial as ext from either 5000 or Axxess.
If there are two many to do this, use just one phantom that fwds to SIP dial tone (ext 7 = SIP trunk access code + pause). Then you dial 7 from either system, followed by specific ext.