Is is possible to conduct SSL-offload aside from port 443 or 9443 like 26300 etc...

Yes, you just create a virtual server with port 23600 (instead of normal 443 for ssl) and attach client ssl profile to it, and any other profiles and configurations as you would do with normal port 443 ssl-offload. Pool member can be with port 23600 as well (or any other ports you like). Pool member obviously listens to http if you do ssl-offload at BIG-IP.

Thanks Thi.