You better be
very sure that the problem was indeed caused by a faulty flap position indicator before attempting to extend the flaps with the alternate system. If the indicated flap asymmetry was genuine, the alternate flap extension system could easily worsen the situation. The TE Flap Asymmetry checklist warns against using the alternate flaps switch for further extension of the TE flaps for a reason:
CAUTION: Do not attempt to move the trailing edge flaps with the alternate flaps switch as there is no asymmetry protection.