the solution is symple: use etable data in this way:
ETABLE,S1,S,1
Etable,s2,s,2
etable,s3,s,3
SADD,S12,S1,S2,1,-1, ,
SADD,Sisochr,S12,S3,1,-1,
with this solution the problem is solved but does anybody knows if there is an ansys function to do this?.
thanks