More of an add-in I think. You can install TotSC and it will immediately raise the exp pt cap to 160,000+, and add the new areas that come with the expansion. I don't know whether it adds new weapons, armour and items but I suppose it would. From what I gather you can, if your party is high enough in levels, procees to explore these new areas, and go back to the main BG1 story line at any point.
I'm not sure whether beating the boss at the end of BG1 ends the game, or whether you can continue on with the other areas in TotSC, without having to start a new TotSC game.
In IWD for example, beating the boss monster at the end of IWD *ends* the game, and if you had HoW installed, and hadn't started it yet, you could only do so by starting an "expansion-only" game. I did that, and lost a number of useful items like scroll cases, gem bags, etc. And their contents!!
[ 02-27-2002: Message edited by: Hayashi ]