I am not sure how I would be able to detect what components belong to specific Mods (such as CEP2), so at the moment, I can’t see how to automatically create Mods from already installed components.
You can share Mods by exporting the Mod on one PC and then Importing the exported Mods on another PC. I have not tried this approach between different users logging onto the same machine, so I am not sure how that might work (assuming each user has different NWN and NIT installations). You can also have multiple Profiles and export and import Mods between the Profiles. Each Profile can specify a different NWN installation (for Beamdog’s EE, you can also specify a different User Files folder).
Instead of using Add to include already installed files, you might want to Create a Restorer and then copy the files for each Mod to NIT’s Mod Folder. This is described in the Help (Deal with existing installed Mods and Create NIT Mods from Restorers).
Another approach you could use is to create Mods from installed content is to Create a Restorer, then delete the files you want for your Mod from the Restorer’s Mod Installer folder. Having done this, use Create Restorer again to create a Restorer that has the files you want to include in your Mod (ie the one you deleted from the previous Restorer’s Mod Installer folder).
Add should default to the folder you specified on the Locations page in Settings. The default should be your browser’s default download folder (usually C:\users\UserName\Downloads). If it starts at C, it probably means that NIT could not determine your Downloads folder location. Use Settings to specify where it should start.
Add is intended to always start at the folder specified in Settings (because, usually, this where you download files from the Internet).
Hope this helps.