You can add .net class libraries to MSSQL. This will allow you to do anything that you can program. And you can definitely make a .net class lib that can query Active Directory
However! The only reason I can think of doing this is that you completely lack an application layer to your software and are doing everything in stored procs.
This is considered bad practice for many reasons, but most of all because code running on your SQL box slows down its primary function of being a database and makes it an expensive bottleneck to your entire operation.
Edit: As you updated I've thought of a second reason:
You have implemented change control on your source code, which means that if you change the source code you have some red tape to go through, perhaps versioning, testing and deployment. However, you have not implemented the same controls on your database and can update stored procs or data without having to do these things.
This is also bad practice, as you should implement the same change control for your database as you have with the rest of your code.