Yes, you can use a Y-cable that has two female mono jacks and one stereo plug, plug the stereo plug into the K&K and blend away. You will need to run short patch cables from the pickups to the Y-cables.
When I was blending two pickups I had the K&K velcroed to my amp, and used a Dean Markley stereo cable:
http://www.musiciansfriend.com/produ...ble?sku=330237
That way I didn't have to use patch cords, y-cables, or mount the K&K to my bass. The K&K has plenty of gain to power a long cable. This cable worked well (considering it's low price) and I didn't notice any loss of tone. All I had to do was plug into my two pickups, & plug into my K&K. It will take some time to set your gain and EQ levels, but when you find your settings it works well.