The output of the boiler coil is probably already plumbed with a tempering valve to keep output temps reasonable. (If it isn't, it SHOULD be- 180F water is dangerous!).
Electric tankless heaters modulate to maintain a constant output temp, so there shouldn't be any issues with putting it in series with the boiler coil's output.
But they DO take a whole lot of electrical current. Many/most retrofit installations of electric tankless units require a power service upgrade. Talk to your electrician first. It may be cheaper to install an indirect-fired tank off the boiler, which will be far more efficient than using the internal coil and more efficient than a standalone gas/oil fired HW heater. Proof: http://www.nora-oilheat.org/site20/u...ciencyTest.pdf
In most markets an indirect-fired HW heater running at 50-70% summertime efficiency will be cheaper to operate than a 99% efficient electric tankless during the summer, and will improve the annual numbers for the heating system as well. (If it's a reverse-indirect like a TurboMax or ErgoMax plumbed as a buffer for the heating system you typically get double-digit fuel savings out of the boiler in heating mode too.)