How can I transition to another domain without leaving current domain?

I am in IT sector, an Infrastructure Architect. I am currently an SME in my domain and able to find plenty of challenges in the same. However the current job market expects everyone to be multi-skilled.

Even though my manager is semi-supportive for me to switch domains, my current responsibilities are over my head to even think about next one. With recruitment frozen, I do not know if I should train someone to take over my responsibilities (cutting my branch) or to just do extra work on other domain (additional responsibilities without visibility/pay).

Looking for advice from people who may have walked my situation.

Thanks