Nope. The Control In is only for an external footswitch or expression pedal. Not for another audio input. If you use Y connector, you might be able to make it work into the Input...maybe...but not the Control In.
I suggest getting a small 2 channel mixer. They're $30-$50 bucks new.
but you CAN practice with a computer and the B2.1u...
since the B2.1u is an ASIO device, you can play .mp3's via WMP or similar and play along with the zoom. I do this all the time.
I simply plug the zoom into the USB port, boot the computer...plug headphones into the zoom's out, plug the bass into the zoom's input, and play along to my .mp3's...
I use a laptop for this purpose and this is my main method of practice.