Consolidating different charts of accounts structures

I'm running into a few challenges with the consoldiation mapping rules (GCS) and was hoping someone may be able to help out.

1st challenge:  The target (parent) company has fewer general ledger accounting flexfield segments than my source (subsidiary) company.

Parent company has only 4 segments, subsidiary company has 6.  Mapping rules seem to be driven by parent segments.  Is my only option to define Account Rules?  Is there a way to use the Segment Rules?  One thought I had was that I could add the additional segments to the parent COA structure and set them to a constant value of zeros.