OpenRA

I'm still relatively new to map making (outside of using the classic Westwood-made editor packaged with RA Gold) but, in Tiberian Dawn (C&C), can Nod obtain access to the Ion Cannon if they capture an Advanced Comms Center (ACC) in a match with Unrestricted authorizations?

I know in the actual campaign there was that whole process of getting the codes and going to a specific facility or whatever but, I'm talking about in mutliplayer and skirmish because I'm trying to make a map with an ACC as a captureable structure.

Yes, capturing an Advanced Comm Center will grant you the Ion Cannon. So you should be alright with your map.