I've thought about adding a zone, so I've given this some thought. I wanted to split the floors into two zones, so both would be a similar size. In your case the basement zone would be much smaller then the rest of the system, but the water must have a minimal flow rate through the boiler. You would probably have to use a pump which is more powerful than needed, and larger plumbing to reduce pumping losses. In this case the basement zone's pump should be plumbed in parallel to the existing pump, and maybe a one-way valve before or after each of the pumps. Each of the thermostats (one for the basement and one for the rest of the house) should be wired to only one pump, while their "ON" relays should be wired in parallel to turn the boiler on.
There is a second solution, which requires more hardware, but is more functional: Get (make) a small buffer with two pairs of inputs/outputs (not sure what it's called in English, translation from Polish is
hydraulic clutch)
Hook the boiler up to one side, and the rest of your system to the other. Each zone on the system side has its own adequately sized pump (plumbed in parallel), as does the boiler side. This small buffer allows the water to flow at slightly different speeds on each side. If you have enough space, you can make the buffer slightly larger to hold more heat (a
heat accumulator/battery is a larger version of this). Each t-stat turns on the boiler pump and its relevant zone pump. If the water flow through the boiler is faster than the system side can use the heat, then the boiler should adjust its burner's flame size and/or cycle on/off to keep the water at a set temperature.
I hope this was helpful (I'm not totally awake yet
). I can make a diagram to show the set-up if needed.