When I add Subs I use either VSO or WinAvi. As long as your subs have the same name as your movie and are in the same folder, either of them software programs should add the subs.
In the past i've burnt an Avi and SRT as data and played it on my divx player and that worked, it only works on certain divx players though apparently and obviously ya have to have a divx player in the 1st place, lol
I know there is a way of addin them with IFoEdit but Ive never done it so cant help ya there.
Try this site for info
https://www.doom9.org/index.html?/mpg/ifoedit-basic.htm
And this site if you can't get decent Subs
https://subtitles.images.o2.cz/
Hope that helps m8