How to manage one heat zone with two thermostats?
With one zone, there's only one thing that can be controlled by a thermostat: how much hot water is pumped through that zone. For example, if your main house is a little cold, there's no way to add heat to the main house without also heating up the addition (and vice versa).

If you're ruling out adding more zones, or somehow adding controllable radiators, then all you can do is balance out the heating effectiveness between the two areas. It sounds like for a given level of heat, too much is pumped into the main house relative to the addition. So, you need to either a) increase the effectiveness of the radiators in the addition, or b) decrease the effectiveness of the radiators in the main house. My guess is that you'll have to do b), which you can do by closing radiator louvers, blocking the air path, etc.

If this is done properly, and you don't change the heating needs of one space relative to the other (e.g. don't leave the addition's front door open) then you may be able to get the temperatures to be at least close to each other.