Using a alternative port for a GMS client

I know this question is not directly GMS, but I have been searching Google and Apple without finding the answer.
I want to have GroupWise web access on default port 443 - and I would like to use GMS to get mail and calendar on iOS clients.

So I will make a NAT rule in the firewall translating an alternative port to my GMS server on port 443.
Can this alternative port be configured in an iOS mail account (Exchange type)?