Can I procmail to existing mbox files?

I've been using Netscape Communicator's message filters so far, and want
to move to procmail.  But, from what I've seen, if I write a procmail
recipe that points to an existing mbox file, it creates new files with
some oddball name instead of neatly appending the filtered messages to
what's already there.

Is there any way to make it work the way I want?  It would be a real
pain in the you-know-what to have to manually move a bunch of mails
around... :-)