Add site collection administrators without access to site collection?

I've been given a SP server to which I can RDP (member of administrators group) have access to CA but not a site collection I need to get into. It's not AD or on our network. My account authenticates but doesn't authorize (Error: Access Denied, you are currently signed in as domain\myusername, etc), I can't see any content. I could use "Change site collection administrators" but I don't want to replace the primary/secondary accounts and block anyone else out. Safe to add another via powershell?