It's a matter of common courtesy to ask someone if you can use their existing stuff in your mod rather than doing it without permission and then going "I hope you like it!" when they finally catch on. Some people might even consider it thievery.
Also, you can't really "merge" area models like you're wanting to do. At best, you'll maybe, maybe be able to turn it into a placeable, but it wouldn't have any walkmesh and your character would probably fall through the module or something while trying to enter it. It is NOT POSSIBLE to edit healthy, working new area models at this time, as has been said on these boards a thousand times to a thousand different people who are completely unable to grasp the limitations of either the engine or the modding community. And if you're coming on here two or three times a week to ask the same questions that have been answered in the stickied tutorials throughout the board, I'm going to go out on a limb and say you're not going to be making KOTOR history and creating a brand new walkmesh for your super-duper "merged area model."
First off you might not know this but I am part of the team that is making the mod that level is going into. So I have rights to use it to my own liking, that is why I said "I hope you like it". Also about the model, I know it has been said it is impossible and I respect that BUT I am not making a model that you are going to be able to walk into and I know that has been done and I also know that it is possible. So basically what I am trying to say is that the model that I am trying to add on to that other model is going to be for show and is not going to actually be used.
P.S.: Also if you don't believe me when I say that I am part of the team that is using that mod or I have rights to use it, check out this site and read the first post, it will confirm that!:D
Link:
http://www.lucasforums.com/showthread.php?t=168281)