PF source nat rule for ftp?

I have a freeBSD box that I'm using to gateway to the internet from my private network. Also, I'm trying to use frox to transparently proxy ftp traffic (because it has caching). After finding this post: http://www.clearchain.com/blog/posts/frox-pf#Frox_PF_Transparent_Proxy_Patch, I made the changes recommended and it worked, mostly. Some clients had an issue with the ftp data stream coming from the router and not from the actual server (specifically ncftp). Is there a way I can NAT the data stream's source address so that it looks like it's coming from the original source address? I'm still fairly new to PF so any help would be appreciated.

Thank you, in advance.
 
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