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You won't actually get Altium support here but someone may know what the issue is: CircuitStudio is really old now and hasn't been updated in a long time.  At this stage I'd be looking at alternatives such as KiCad which is very well supported and from what I can tell is more feature rich than CircuitStudio.  

The issue is that I got a circuit studio PCB file with a layer that's not supported in Kicad so I want to open it to verify the Kicad conversion isn't missing anything.

Ah, not sure what to suggest short of contacting altium themselves.  Somebody here may have an idea how to work around the actual problem if you can give more details.  Hope you manage to get it sorted.

It's an iterative process with any import from one PCB CAD tool to another, so what you could do is see the list of unmatched layers, and set them to your best guess layer in KiCad, and then you can see what that layer looks like, so you can go back and reattempt if it is clearly identifiable as something other than what you originally guessed. Then, run design rule checks, generate Gerbers, and manually (visually) examine or compare them closely (obviously, this is easier if you have Gerber files from the original CircuitStudio files). If things look wrong, you'll then have enough knowledge to restart, and try a different mapping. It should be less than a day's job unless it's a very complex board with more than four layers or so.

Note that there are automated ways to examing the Gerber files too; ViewMate software can do that (it's very good), although it might need a paid option (but it's not expensive).

It's an iterative process with any import from one PCB CAD tool to another, so what you could do is see the list of unmatched layers, and set them to your best guess layer in KiCad, and then you can see what that layer looks like, so you can go back and reattempt if it is clearly identifiable as something other than what you originally guessed. Then, run design rule checks, generate Gerbers, and manually (visually) examine or compare them closely (obviously, this is easier if you have Gerber files from the original CircuitStudio files). If things look wrong, you'll then have enough knowledge to restart, and try a different mapping. It should be less than a day's job unless it's a very complex board with more than four layers or so.

Note that there are automated ways to examing the Gerber files too; ViewMate software can do that (it's very good), although it might need a paid option (but it's not expensive).