Hibernate version: 3.0a

I have two legacy tables:

table A:
composite_pk
pk_A1
pk_A2
/composite_pk
prop_x
prop_y

table B:
composite_pk
pk_A1 <foreign key from table A>
pk_B2
/composite_pk
prop_w
prop_z

Table A and B are two Java objects: A contains a set of B objects retrieved through pk_A1.
Hibernate does not allow me to specify a <set> since it requires both columns from A's primary key.
Since the DB cannot be modified and no tables can be added, I tried quite a few mappings but I'm always stuck...
Any suggestion would be appreciated.