NAT 1:1 question

opa.at

hi guys,

i hope i don't do dual posts, i have tried looking around the forum - and all the rest of the internet for that matter :)
but ether there are not many people that need this or i don't come up with the correct keywords to search for ::)

following scenario:

external subnet
|
|–-----server1
|-------pfsense------lan------clients
|----------opt1----clients

what i would like to do is that i want to move the server1 behind the pfsense BUT i can not configure server1 :)
so i was thinking of giving opt1 a virtual ip, the same ip as the router of the external subnet, and than hookup the server1 to it... but i am sure this will compleatly messup the routing of the firewall... right?
so, is there a solution for this? i have to add that i don't have to many options to do testing so this is why i am sharing my thoghts before actually testing it.
the reason why i want to do this is thati want to monitor the network usage of server1, QOS and also i want to limit it's available bandwith.

thanks for any informations!

opa