Yes you can add RBP to an existing game. There's an update console command you may have to use but that's about it.
Also for anyone reluctant to use LAME and RBP:
Testers are needed for a No requirements, No gameplay changes version.
http://www.moddb.com/mods/integration-the-stranded-light/news/testers-needed-for-a-reduced-requirements-version-no-more-rbp-or-lame
Anyone interested can PM me, bg2408, or post in the http://www.gamesas.com/index.php?/topic/1184421-relz-integration-the-stranded-light/.
The Underdark mods...Well.... :confused:
Spoiler Viconia came off as a bit psychotic, forced, unraveled, and just plain awkward and bizarre. I played Viconia part 1 all the way through and part 2 about 2/3 of the way through. Now it's not that she was meant to be all of those things, because that would be fine, it's rather that her character, personality, and dialogue are poorly developed thus making her seem crazy, fake, and unrepeatable. For people that don't really care about character development or personality this prolly isn't a issue for them at all because; hell "I got a hot chick companion who when she's not fighting along side me she's wanting to grind all up on me!"
Can't really speak for any other underdark mods because I've never tried them. Viconia was based off of/transferred from a Baldurs Gate mod, so that could be part of it. The other companion mod is male, and I believe it's the authors own intellectual property so it might be different.
Windfall has a good story and characters
Malevolent has pretty good characters like all of Simyaz's mods
Lore Dialogues 300 allows you to ask NPCs a bunch of random things like in Daggerfall/Morrowind.