Using SFP port as WAN2 on 100D

Here' s my situation: I set up the WAN2 interface to prepare for a secondary Internet connection. The new Internet connection turned out to be fiber, so now I' m going to use an SFP in Port 15 on the 100D for the secondary WAN connection. What I' m wanting to do is assign port 15 to the WAN2 interface, so essentially the WAN2 and Port 15 physical ports would both behave in exactly the same way: if I wanted to use a copper connection I' d plug into WAN2 and if I wanted to use fiber I' d plug in to the SFP on Port 15. My questions are: 1. What is the best way to go about this? I would have assumed that I could just add Port 15 to the WAN2 interface/zone but it doesn' t look like it works that way. 2. Are there any security concerns with using port 15 rather than the dedicated WAN2 port for an Internet connection?