Flat handlebar caliper brake compatibility I am trying to convert my flat bar bike to drop bar. I have bought STI's but I also want to use additional brake levers(like cyclocross bikes, flat bar levers) as well. 

I may be wrong but cable pull ratio of caliper brakes(which I am going to install on my bike) must be different so I think I can't use my old levers with caliper brakes. This is my first question, V brake levers can be used with calipers?(I think no)

Now I am looking for a suitable brake lever. But how can I understand if I can combine them with STI's and caliper brake? Do they have a name like "Flat bar road brake system"? I have taken a look to some bike stores on web, they only tag "v brakes" and "road brake shifters".
The product you are looking for is interrupter levers. regular flat bar brake levers aren't designed to work in conjunction with STI levers.  The ones I linked to specifically say they are compatible with caliper/cantilever brakes.  I'm pretty sure most interrupter levers would be compatible with caliper/cantilever brakes because v-brakes don't tend to be found on bike with drop bars, which is where one would normally need interrupter levers.