Insert subtitle track into dvd .iso on a Mac? Freeware prefered.

Okay, so it's driving me nuts. I google, and google, and google, and always come up with the same tutorials for Windows... Except I've got a Mac.

Here's what I'm looking to do:

Somebody was kind enough to give me the .iso for a dvd.
I'm going to make subtitles/captioning for the dvd (in what ever format they need to be in).
I want to retain the dvd menu and insert the subtitles so they play automatically without altering the dvd menu or embedding them into the actual video.
Then burn the dvd with the subtitle track to play in dvd players.

I'm not looking to add the subtitles into the video or make an mkv to play on my computer. I know how to do all that. It's the making it into a dvd with subtitles I don't understand. ==;;

I'm completely new to all of this so please use very simple terms or be patient enough to explain things I don't understand.

Thank you to anybody who can help and I hope I'm in the right part of the forum.