I don't think you can, you've gotta have them hardwired into your PSU cables. You can use two normal PCI-E 6 pin's instead in a HD2900 though. (I think?)
what's your power supply? some supplies (eg, Corsair Hx620w) have the juice, so Corsair simply changes both modular 6 pin PCIe cable into 8 pin ones
My Corsair 520W has 6-Pin + 2Pin connectors, so you can use either 6 or 8 if thats what you're talking about...
you can, I saw them a while ago; and I have 2 6pin in mine. The 8 pin is to give it overclocking juice I believe.
the extra 2 pins are for ground only, this increases the max current draw from 75 to 150w. (or so i have read) only place i have seen the adapter is a US store. my pro only has the 8 pin socket - not 1x6 and 1x8
Also, if you've got a Corsair modular unit, give them a shout and they'll send you some new, updated modular cables that have one plug on the PSU go to one 6 pin and one 8 pin. Got mine today, they're fantastic.
For the older, more power hungry 2900XT, have a look here http://www.bit-tech.net/hardware/2007/05/16/r600_ati_radeon_hd_2900_xt/5 rather informative, but basically echoing what people have said already
i haven't tried buying one yet, but i have the link bookmarked still http://www.frozencpu.com/products/6..._Adapter_Cable.html?tl=g11c28s427&id=wv5fCFb6
$10 then $30 shipping! i think i would rather get a new psu.. if you zoom in on the image in thaat link you can clearly see that it is only an extra 2 ground pins. I may and it is a big may.. cut my other 6 pin connector up and slap 2 ground pins on.