You could total do this depending on how your zones are currently set up. Keeping everything safe is the important part. A couple concerns. First, if you screw up the plumbing and air gets trapped in your boiler, you could create steam, steam creates too much pressure, too much pressure causes your boiler to explode and take your house with it (assuming cast iron boiler, no pressure relief, no temp/low water shutoff, etc). Second, 180 degree water will scald you on contact. Make sure all connections are solid and all valves and drains are safe.
If my name was Bubba I would tap into a supply and return line with 1/2" ball valves and run 1/2 oxygen barrier pex to a big coil of copper in an HLT. Open the valves and turn your thermostat up whenever you want hot water.