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I haven't had a metro trans apart, but with enough ingenuity and motivation it is possible to extend transmission shafts... it's just not easy. It there were a way to drive another gear off of 5th, employing yet another shaft, you might be better off. Again, I've never had one apart. I have extending a couple of precision shafts for a hybrid car back in 1982 or so, so I do have some real world first hand experience with that.
I don't twant to sound like a wet blanket, just look at ways to overcome some of the potential issues I see coming your way.
In most cases, those transmission shafts are hardened, very straight, and have presicion ground surfaces on them. What this means to the modifier is drilling, boreing and cutting a keyway might be out of the question unless you aneal it first, then reheat treat it later <hold that thought>. Another way I have heard of, some company that adds a 5th gear to a 4 speed box, is to machine the shaft, make an extension, then vacuum furnace braze the two together. Then re-heattreat it. I understand it was quite a development project for the 12 person company that did it.
Then problem with reheat treating any previously harded shaft is several fold. You need to tell the heat treater exactly what the material is so he can apply the correct recipe for that specific steel to either anneal, or more importantly re-heatreat it. If you work with a heat treat house beforehand, they can hardness test the shaft, so at least they know what to shoot for in the final product. Another issue is you usually get scale that ruins the precision ground diameters that ride on needle bearing (some transmissions) or need to be very precise diameter to accept a press-fit bearing. The shaft usually bends.
Grinding the surfaces undersize, building them up with hardchrome, regrinding and straightening are all possible, but unless your brother in law owns a grinding shop, bring your checkbook AND your Mastercard... you'll need both.
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