I imagine you would need to use SQL to accomplish this since AD is not really a "data source" in the traditional sense. I wrote a post way back explaining how we created a SQL "interface" for AD and there's two approaches:

1. Use ASDI to build stored procedures and such to retrieve your data
2. Create a scheduled SQL job to periodically pull the data and refresh a db that you then use as your data source.

With either of those, you can then connect them to Forms as a data source to use in form lookups.

(Manager was one of the first ones we tried, and something to note is that there's a lot of relational structures in AD that you don't typically see on the front end. For example, manager is stored as the Distinguished Name, like a reference, so if you want the managers full name, display name, etc. you need to pull it all together like a JOIN).

Active Directory as a Forms Data Source - Laserfiche Answers