I have a three tier hierarchy | ConfigMgr 2007 SP2 R3

CEN 

\

Primary A | Primary B

                      \

                  Primary XY

Addresses are configured as follows

CEN > Primary A

CEN > Primary B

Primary B > Primary XY

My primary site XY is having issues replicating slowly from Primary B.

Can i add an Address from CEN > Primary XY and what are the implications ?

It may let you create the address but it won't ever get used unless you change the parent/child relationship.