If you want mids I think you either want a whole different speaker configuration, or you need to add a 410. A 210 wont really do much. You can match the impedance by getting a 4ohm 210 (an 8 ohm one not only would send a confusing message to your head,which wants to work with only a 4 ohm or 2 ohm load, nothing inbetween) but the 210, being 8 ohms, would probably not even get as much power sent to it,and it is already a small speaker configuration. Therefore you probably would barely notice a difference.
a 4 ohm 410 might work just fine. I would place the 410 on the ground and the 215 on top. Take some of the boom out of the 215 and add some floor coupling to the 410.
Other options: sell your 215 and buy a 410. Or a 212. Or something like that.
I'd rather be playing my Matt Freeman sig