Candas,
Perhaps you can swap the ref set for 'entire part' and utilize existing dp's/da's to constrain the part. Then, swap the ref set back to what it was and save the assy. Of course, this will only work if there are the appropriate existing datums in the read only part... which is yet another reason why I always utilize a datum CSYS in the piece parts, but this is diff thread...
Regards,
SS
CAD should pay for itself, shouldn't it?