Append variable to file variables

I would like to use the whole contents of a JSON file as my environmental variables. I am currently using environment: ${file(./${self:custom.currentStage}-env.json)}, but I would also like to add a variable that I am getting as an option. I have added version: ${opt:version, ''}, but I still need to append this to my current list of variables. Is there a good way of doing this or do I have to separate every environmental variable individually? I am using the whole file system as it’s more convenient and I have a lot that I would have to add then update when I add a new one.