Rows in GridView dynamically call a ModalPopupExtender

Try doing a FindControl("mpeFicheInd") to get the Extender, then you can modify the members of its TargetProperties collection. The XxxProperties aren't really controls and I'm surprised FindControl even finds them above. (Hum, actually it doesn't, I guess - hence the build error.) This should work for you as the general technique's used by many.