How do I add a "c" wire to the boiler for a Nest thermostat?
Yes, that C connector on the valves is what you want.  The nest thermostats use very little power, so that shouldn't be an issue for your transformer.

There are two wires coming from the transformer.  On a typical HVAC system, the red wire is used for control, and the other wire is what gets used as the "common" or C-wire.  In your diagram it looks like white is being used for control and red is going to the C terminal of the valves.