Forward a UDP port range across a couple of addresses

Hello;

I've got 3 networks that need to speak to each other. 2 of the networks
are connected via IPSEC gateways, which is working fine. The 3rd network,
due to equipment constraints is not reachable from 1 of the networks... ah
hell, it's probably gonna make more sense in a diagram.

All networks are 8 bits wide (255.255.255.0 netmask).

Network A Network B
(10.1.1.x)-----IPSEC---(192.168.1.x)
|
|
Host Z
(192.168.1.222)
multihomed
(192.168.254.222)
|
Network C |
(192.168.254.x)-------------|

an IP on network A needs to talk to 2 IPs on network C, on a range of ports
from 4000 to 5999. I CANNOT attach the devices on network C to network B.
Can I use iptables and numbered sub-interfaces to "forward the IPs"?
essentialy can I make the 2 IPs 192.168.254.2 and 192.168.254.3 appear as
192.168.1.6 and 192.168.1.7, but only for ports 4000 to 5999?

Please help... I think I may have reached my limit with this one.

Thanks

Jeff