Hello! I am planning to buy a signature patch. I wonder if I can still load it in the Omnisphere I got from our sister website. Thank you!

 

Omnisphere soundsets have no copy protection or digital rights management, so yes, you can use legally purchased soundset with sister site Omnisphere.

By the way, you should contribute to the community by supplying that soundset to the sister site. You can upload it yourself or contact a moderator and they can do it for you.