I found a way to do this and it is possible, and it's surprisingly easy, so you may know how to do this already. In the QC file, look for something like this:
$bodygroup "sleeve"
{
blank
studio "brit_sleeve"
studio "axis_sleeve"
}

If it's an American weapon there will be a 'studio "us_sleeve" and a blank where brit_sleeve is. So all you need to do is replace the blank with the correct line of code. For British weapons, enter this:
$bodygroup "sleeve"
{
studio "us_sleeve"
studio "brit_sleeve"
studio "axis_sleeve"
}
And copy the us_sleeve.bmp and us_sleeve.smd files from a decompiled Allies weapon into the folder and compile, and do the same for British weapons. Axis weapons have all three sleeves avaliable, so there's no need to change it.

However, if there's a map where you can pick up another nation's pistol or melee, you're not going to see a sleeve unless you have all three nation's sleeves in the QC pile and recompile.

Nice work! :D This'll be a lot easier than replacing them all by hand.

The British and US don't play any of the maps at the same time, so why would you need both sets of sleeves, where as the Germans do because they play ever map. Or is there some custum maps that allow the US and British to play together, although i've always thought this was not possible in dod v.1.0.

you can place british guns in maps with americans, and vice versa