Adding subtitles to a retail BD (or using external ones)

I've been consumed with GITS S.A.C lately, and having seen a blu-ray rip with subtitles added to it, I decided to stop before 2nd GIG and buy the series on Blu-Ray, wanting to support the show but not wishing to give the image quality up. After much research, I found the only BD release of the full series is spread across 4 box sets, with dual audio and Japanese subtitles. Knowing this as someone who dislikes the English dub and despite their far from fetching price I purchased them.

I know there are ways of basically authoring the BD onto a new disc having added subtitles to it on my PC, however I'm not interested in that; I want my retail discs to be the ones I use.

Is it possible to add subtitles to the original disc? I don't care how long the process is if it can be done. If there is really no way to do this (and an explanation beyond 'write-protection, sucks2bu' can be provided?), then could someone explain a way for me to play the original disc and use external subtitles? Again, I don't mean creating a back up and adding external subtitles to that in a player. Lastly, if I have to go with the latter option, does anyone have any tips on how to make this an enjoyable/quick experience such as media profiles to avoid having to add subtitles every time a new episode starts?