As Carl pointed out, you can wire the lowers as you describe.
You could tap either the front speaker outputs, or the rear speaker outputs from the H/K to feed the J&M amp - then hook the lowers to the amp's speaker outputs. Since you are using 2-ohm speakers in the lowers, you can only run the lower speakers from the amp. After you have the lowers installed and hear the output level difference of the lowers compared to the stock speakers - you may be tempted to wire the front or the rear stock speakers in parallel with the new lower speakers.
Do not attempt this as it will drop the load on the J&M amp to 1-ohm and more than likely the amp will either go into immediate protect mode or will over-heat. You could, however, wire an additional set of stock speakers to the new lower speakers in series. This would create a load on the J&M amp of 4-ohms and the amp should work fine.
Good luck with the install ...