I'm going to give this a try and I guess the worst that should happen is that I have to call a plumber in the morning.

Let me know if these steps look correct.

Leave check valve in place -
1. Turn off power to boiler system (leave recirc and house water on)
2. Drain domestic hot water line
3. Turn off house water and hot water recirc pump.
4. Drain water tank and water lines
5. Install expansion tank between check valve and water inlet of hot water tank
6. Turn everything back on in reverse order