Frankensteining 101:

1. Make sure the new neck and the new head use the same internal names as the old neck and head: "neck" and " head".
   That means, every vertice of the new head must belong to a group called "head" an every vertice of the new neck must belong to a group called "neck".
   Use Poser's group editor to check and eventually fix this.

2. Export Apollo's body without the original neck and head as an object file.
   Don't forget to completely zero all joints and morphs before you do that.

3. Re-import that headless body into Poser.

4. Import the new neck and head into Poser.
       (Check point 1. with the grouping tool!)

5. Export both Apollo's body object as well as the
       new head and neck object simultanously from    Poser so that it will be fused into a single new object.

6. Edit a copy of Apollo's cr2 so that it calls the new combinen object (Old body + new neck/head)

7. Delete the now useless head and neck morphs using morphmanager or similar.

NOTE: Unless the new neck has the same UV-mapping as the old neck, there will be a visible texture seam where old and new parts (chest and neck) meet.