Get a large pot and boil water in it. Then submerse the ABS (with some out so you can get it back) in the hot water and let it sit for a little while. Pull it out and mold it by hand.
Try it with junk ABS first so you can get the hang of it.
That's what I'd guess, it works for PVC. Don't know about ABS though.
Anyway, I'd suggest using PVC and replacing the parts that have to be modified; it's more rigid and therefore will take more of a beating, not to mention that it is easier to soften and work (again, educated guess, as i have melted ABS, but never actually worked it.
. . . But either way, you would need a heavy-duty gas mask, high levels of ventilation, and something like a kiln . . .
you wouldn't need a gas mask if you melted 10 pounds of the stuff, as long as you've got the also-mentioned high levels of ventilation.