How to define a primitive device in Proteus?
It's in the Proteus Help file:

SCHEMATIC CAPTURE HELP (F1) -> MULTI-SHEET DESIGNS -> Hierarchical Designs

See the section "External Modules" - you should be able to define the child sheet as a .MOD file, then use the Make Device command on the parent component and use include the .MOD file there.

I've never done it before, but I'll try it now. Sounds like it should be simple...