Would it be possible to add an option to pause the game outside of the sandbox, while in a sandbox?

Unfortunately, the modding APIs do not provide a reasonable way to do this. I believe that other mods have attempted, but I don't think anything has been successful.

Ah okay I understand.
I saw an option to pause the game, while using the Editor Extensions mod, so I thought it might be possible. Or is this a native Factorio option and not being provided by Editor Externsions?
https://imgur.com/cJS3aj9

I think there is a good reason why the API doesn't provide a way to pause the game outside of the sandbox: Your request may make sense in single-player mode -- but in multiplayer, if one player would enter the sandbox, this would pause the game for all other players. :-)

While I could provide that same functionality that you saw there, that pauses everything, not simply "everything but the sandbox."