If you want to add a placeable via the GIT file, you'll want to extract and edit m01aa.git from end_m01aa.rim. Open the file with K-GFF, duplicate one of the existing placeable structs, then edit the details. After you're done editing the file, you'll have to compile it into a .mod and install it to the modules folder. Extracting all the files in end_m01aa.rim and end_m01aa_s.rim, put in your edited GIT, and then create a end_m01aa.mod file with ERFEdit and add all the files.
If you're planning to release it as a mod, though, you'll want to configure it with TSLPatcher or else it won't be compatible with other mods that edit that area.
If you add the placeable with a script, you only have to edit one file and install to Override, so that's somewhat simpler provided you understand the NWNScript language. The best course of action in this case is to find and edit an existing script, either the OnEnter script for the area (k_pend_area01) or a script that fires early on in the opening cutscene (such as k_pend_rumble01).