Is it possible to use the ideology editor to just edit an existing ideology and add subideologies or no?

Yeah, you can do that.

Create an ideology with the same name as the one you're editing, e.g. Communism, and then in the tool choose the "Import" option in the sidebar menu.

If you're unsure of the file(s) needed, you can hover over the (i) icons to get a path to where you can find the correct file for each input.

Thank you!