just bought some GFS retrotron pickups, 4 wires.

The problem is im left handed and the wires come out of the back of the pickup at the wrong end, the end furthest away from the hole in the cavity on guitar (SG) (they would be next to the hole on a right handed model) and so have to be folded underneath the pickup to reach the hole. Because of this the wires arent long enough to reach my tone/volume pots.

Any problems with just soldering bits of new wire onto the original wires?

Or fitting the pickups upside down?

up the bracket
Shouldn't be much of a problem. Usually pickup wires are wrapped in a thin metal sheathing to cut down on noise. If you have to extend them much, that could present a minor problem.