Unfortunately, it looks like there is no way to include a comma in the output, at least as much as I could tell from googling around.
I don't think that tbl is the greatest option for your use as future research scientists as the code is a bit clunky and there is not a lot of help info out there. table1, tabout and outreg are all better options
That being said, it is fine to use for your final project.
If you want to include a comma in a Markdoc table, you can use markdown syntax to include a table as a comment. The table can have commas like in the following example, but the problem is that it is not dynamic (it does not auto-populate)
Example .do file here:
https://github.com/haghish/markdoc/blob/master/Examples/tbl/tbl1.do
This website generates LaTeX or markdown syntax for tables. But again, the problem is that it is not dynamic: