Hi all,
finally this is the tip I used:
in the first part I create attributes associated to a sketch curve. These attributes are controled by a relation like:
BASE:CRV_ATTRIBUTES=BASE
with BASE is the name of the attribute and ATTRIBUTES is the name of the curve.
then on the other parts I add a copy of this geometry and I specify the reverse relation:
BASE=BASE:CRV_ATTRIBUTES
and it works!
thank to all for your answers 