The modular cyber hand, Welding Laser, has a 10 shot to deal damage, can it be boosted with a battery pack ? (Like lasers)

Off-hand (no pun intended), I would argue for a no. Sticking a larger batter pack would more than likely cause the limb to be unwieldy.

In arsenal they talk about the backpack...

IMHO "yes but..." : Extra batteries may be integrated in the cyberlimb (definitvely not in the hand itself), but would use up capacity that could find better uses. The best idea is to have a power socket in the hand or wrist, and run a wire from there to an external power supply / battery pack for prolonged operation.

Keep in mind that the tool laser is NOT designed as a weapon. It's short-ranged, poorly targeted and while it can be used as a backup or a nasty surprise I would not use it as the main weapon of one of my characters.

Agreed. If you must use it as your primary weapon, use the rules for an external clip port (Pg. 344, SR4A; Essence .1, Capacity [1], Avail -, Cost 100y) and simply socket a power cable in there. I like it best as a hold-out, though - nothing like getting disarmed by a guard and then giving him a face full of laser.

I'll add that to the Drone. Thanks.