Can I have a separate thermostat for a wood boiler?
You don't offer much detail in your question, so it's difficult to determine exactly what you want to do.  If you're just looking for manual control you could just energize the G terminal on the furnace, which will run the fan in high speed mode (if the furnace has a variable speed motor). Most thermostats have a switch labeled AUTO and FAN. If you put the switch in the FAN position, the G terminal of the thermostat will be connected to the R terminal (meaning the G terminal is energized).

If you want automatic control, you might be able to use a thermostat that has multiple heating controls.  Then  you might be able to use the W2 terminal of the thermostat to turn on the furnace fan, and the outdoor boiler.

Again, without more information it's difficult to tell you exactly what you can/should do.