make explict proxy use the x-forwarded-for header as a source

Hello again,

 It's me again with another weird question. Let's start with the objetive. There are two Fortigates on different sites, let's call "FG-A" the local and "FG-B" the remote. FG-B has explict proxy enabled, a collector, and explict proxy policies. FG-A and FG-B are connected via MPLS.

 On FG-B we want to route certain traffic (Office 365, if you may) to another wan connection, and the rest must be forwarded to the explicit proxy on FG-A. My first thought was to use proxy chaining, but i have a problem: the source IP address that FG-B sees is FG-A, not the user. Ok, that's why we use X-Forwarded-For... right? I managed  to add that header using a web-proxy profile, but my problem is that FG-B ignores this header. 

 Is there a way to make the explicit proxy use the X-Forwarded-For header to take the source IP? Or should i try something else?

 TIA.

PS: Yes, i know that it's an strange solution, but we have some restraints (ie: FG-B is on an ISP, we can just add the other wan connection there).