I have thought about that myself. The routes get very close at one point Here's what I'd do:
--Stay on GR 65 to Navarrenx
--Follow the west side of the river south 7-10 km to GR 78(the Voie de Piedmont) near Prechacq-Josbag
--Continue about 10 km SSE on GR 78 to where it crosses GR 653 at Oloron Ste Marie
The trails are on the IGN topo 25 series map.