Can I run a second cable to add another hot conductor for a range?
Splittng a circuit across two cables is simply not an option.  It cannot be done, and there are big reasons for that, and those reasons are much more critical on high-current devices.  This isn't low voltage DC power: this is AC and a whole lot of it.   

Usually, in situations like this, the barrier is that the cable is too buried in finished walls to economically replace.  You are saying that is not the case for you.  Since you would be adding a cable in any case, add a 10/3 cable.

10/2 to a range or oven site is awkward.  You can get away with it when installing modern ranges which are designed for the world market, and take 230/240V only.  But then you are painted into a corner if you happen to pick a replacement that is North America only.  Perhaps this builder reasoned that it would be easy to swap the cable if warranted.