Shimano integrated shifter/brake: Cantilever compatible with V-Brake?
Generally road caliper and cantilever brakes are not compatible with "V"  brakes levers. You have a few basic options. Find a single shifter and reuse your brake lever, find an integrated shifter for canti brakes,( likely a rare find), up grade your brake to a "V" brake. Your local shop may help with some used parts. In my area used brakes go for around $10. This is for arms, pads and a cable noodle. You should see an improvement in braking with the "V" brakes.