You'll still need to wash it in hot water in a machine, but you'll just have to keep a very close eye on it...like stop the aggitation and take it out about every 2 minutes to see how far along it is. Every two minutes may see like a lot, but some yarn felts really fast. It's the only way to make sure it doesn't go to far.
You could try hand felting it first in a sink with hot water and swishing it around. I'm not sure how well that would work, but it seems like it would take a lot longer that way...maybe easier though since you don't want it to felt a lot anyway.