I'm surprised you find the application unusual. It came up quickly, when I started planning to use DS10A's on my windows!
I found this source:
http://www.smarthomeusa.com/ShopByManufacturer/SHUSA/Item/SHSA1000/
5-packs of magnets for $6.95, but will they work with a DS10A?
Dave
I don't know for sure but I'd be very surprised if they didn't work. Reed relays are very sensitive and I would think that as long as you can mount this extra magnet fairly close to your sensor it should work.
Why not give SmartHome a call on the phone and ask them? They're a reputable company.
By the way I think what you're trying to do is quite reasonable. I did the same thing with the alarm contact on my garage door. I have one magnet in the fully closed position and another in the manufacturer provided, slightly open, ventilation, position.