DaMulta,
If the cards had VRMs made of same exact parts you could do that, but you can't mix any non-Volterra parts with Volterra parts on a reference 4890.
Creating a makeshift 8pin plug by adding the extra pins (which are grounds) wouldn't do anything. If AMD had originally put 6+8pin on the board instead of 6+6pin there would be no gain either. The fact that some AIB put a 8pin on their cards is nothing but a marketing trick.
Electrically 6pin and 8pin PCIe power plugs are identical, the only difference being numbers on the spec sheet in PCI-SIG's basement archive. If a video card requires > 75W from a 6pin plug it will draw it and there's nothing that would prevent it from doing so. The wires themselves, and as such the whole connector is capable of much more than 2A per wire (6A total)...
And as I said, the extra pins on 8pin are just grounds. If 8pin offered an additional +12V plus a ground it might be electrically justifiable. But adding just grounds in the location they are missing from certainly wouldn't make any difference. Maybe if they were closer to the load - like right on the backside of the GPU chip - they might actually do something... On a good day.
To actually answer your question:
Sure, you can add the missing pins.