Can one part of a multi-column primary key be used across tables?
It is not possible to have 'hidden' columns.

When you are using GUIDs to avoid merge conflicts, you must use GUID keys in every table that contains data to be merged.
And when you already have a GUID, it does not make sense to add another column to the primary key.

If you do not want to use GUIDs, just deal with the conflicts, i.e., instead of blindly copying the records, map all old IDs to new ID values.