What I have:

Japanese language BR with multiple M2TS files (its about 49 minutes and is an extra feature), it shows one MPLS file (004) this file shows the multiple M2TS files (006 to 024), the problem is that the original menu allows you to choose these separate files or a "play all" so that explains why they did it that way.

Now, I have actually a full (one large M2TS) file that is the complete 49 minutes and it has an English subtitle file (sup) for it.

What I would like to do but have racked my brain and still can't figure out is of course try to replace all of those separate M2TS files with the one large one with the sup file.

It makes no difference to me all I need to do is add the subtitle file but the only logical way I can think of is to slice up the sub file and add it in separate for each, just thinking that there has to be a better way.