You could try using this guide,
I’ve added subs to an existing DVD,just not to an episodic one…Not sure it’ll work correctly…
You can try by de-muxing (extract the video and audio,and re-encode video with HCEnc to create an .m2v file,then
Use an authoring tool,like DVDauthorGui,and import your.m2v ,audio and external sub,to create new DVD…
I believe(so not sure) with Fab Plat,you can create one VOB,using vob passthrough, and use DGIndex for instance or projectx, to de-mux the video from the one VOB…BTW,you can create one single .VOB with DVDShrink or DVDDecrypter
Perhaps someone else will direct you to an easier method…Good luck!