Is it now possible to assign static IP address to Windows Phone 8.1 to connect to Wi-Fi?

 

This is possible on WP 8.1. although you appear to be looking at it from the wrong end. If your system admin wants you to use a certain address they would create a DHCP reservation on the DHCP server based on the MAC address of your phone. You can get this info from the about section in phone settings.

If you wish to manually set the phone to use a static IP then this can be configured in the WiFi settings for the particular network that you are connecting to. I just looked and it is there on 8.1