business methods with extensibility

Hello,
does anybody know, if it is possible to use dal.start.business.method() with extensibility.
I have declared a business method in the functions section of table extension whinh220.
I want to call this from session extension.

However I get error -2 DALNOMETHOD.

dal.start.business.method( "whinh220",
"update.customs.value",
l.ret,
g.orno,
g.pono,
g.cuva)

Regards
Martin