Hello Andrei,
As Ron said, sure the controller can adopt AP from multiple different rf-domains (sites) remote to it. The most basic of requirements is that the APs at said remote sites be able to ping the controller. If they can't ping the controller then this is something on the LAN side that you need to fix. Once the APs are able to ping (meaning see) the controller, you can select one of the methods listed in this article for layer 3 adoption:
https://extremeportal.force.com/ExtrArticleDetail?an=000081714
I hope this helps,
Christoph S.