Why not

Forum: Linux IP NAT Forum
Re: NAT and masquerading (Doug Kite)
Keywords: NAT masquerade
Date: Fri, 09 Jan 1998 00:58:04 GMT
From: Michael Hasenstein <hasenstein@yahoo.com>

The NAT code comes before any masquerading is done (or afterwards, respectively, for all outgoing packets). If you keep that in mind you might be able to create a useful coexistence.