Dynamic Distribution Group - Filter "Office"

Hi all, 

Apologies, this one is wrecking my head!

We have a set of old DDL in exch2010 who filter the "Office" in users' AD and are now obsolete.

Now, the million dollar question... How do I created a DDL in Exch2013 that will filter the users Office location?

I know I can create a custom attribute, but we have a large turn over, and would like to avoid people "forgetting" to put the custom attribute when the suer is created, when the Office field in AD is obvious, and already part of our user creation process.

Any idea how to include this is the RecipientFilter?

Many thanks, 

Fred