I would have a running Number Field in your other foreign table. Then have a Calculated field that does
ForeignKey & Right( "000" & RunningNumberField ; 3 )
This will get your what your are asking for
This is the key field of the main table.
Not a good idea. Primary keys should be simple, unique and devoid of any "encoded meaning". Doing otherwise risks potential problems easily avoided if you don't use such a field as a "key" in relationships.