Can I use the DJM-S7 to mix music from a smartphone that's connected to the unit via Bluetooth?

Yes.
You can mix music from a connected Bluetooth device by inputting it to channel 1 or channel 2.
*You can configure input routes in Utility mode.
Note: Music played via a Bluetooth connection will be delayed before it's output from the DJM-S7 (this is due to latency).