Quick answer is don't do it .. you will have a constant supply of fresh oxygen-rich water going through the boiler,, which will promote corrosion in the boiler. If you want to use the boiler water, purchase a water-to-water heat exchanger and use it to heat your domestic hot water, or buy an indirect hot water heater for your DHW..
A boiler is an expensive item made from carbon steel. It will corrode quickly with oxygen rich fresh water running through it.. The sealed,and recirculating hot water in the boiler and piping depletes it's existing oxygen then the water is benign.
I use water to water heat exchangers to transfer all my boiler heat into my heated shop floor, and into my home's baseboard/propane boiler system.. the water systems do not mix.. only exchange heat at the water/water plate exchanger.