Can I play DS games from any region on the 3DS?
Yes, what you want to do is possible. While all 3DS software is region-locked, Nintendo DS software is not. Because that region-locking isn't encoded into the DS software, you can play software from all regions on your 3DS.
You can even check out this youtube video where someone demonstrates a DS games from North America, Japan, and Europe on their North American 3DS.