Here's how I added parts from the default 737 to the TDS models:
1.To the model.cfg add: interior=..\..\B737_800\model\B737_800_interior ,replacing the 737 bits of text with the ones from the correct aircraft.
2.Add a texture.cfg to the texture folder; with contents like this:
[fltsim]fallback.1=..\texture
fallback.2=..\..\..\..\Scenery\Global\texture
fallback.3=..\..\..\..\..\..\Scenery\Global\texture
fallback.4=..\..\B737_800\texture (Again, point it to the correct aircraft folder here!)
Or copy the appropriate vc textures manually.
3.Copy over the panel folder, or add a panel.cfg with an alias to the panel you want to use.
4.Edit the aircraft.cfg for the correct viewpoint if necessary.