I don't recommend this, because tankless water heaters have a tendency to get buildup and to clog when put on a circulation pump.
- They definitely do not like hot water returning to them.
- There are some brands, newer ones, that claim to have solved this problem, and the hybrid types, like "Eternal," tend to perform better as well.
"Phoenix" is a highly efficient tank-type water heater that is designed to do what you are asking.
(See the related Q&A below which addresses the Phoenix heater below.)
For more information:
Read Florian Speier's Q&A "Can I configure an electric tankless water heater to run a baseboard hot-water heating system?"
Also, read Bill Bradbury's Q&A "What materials will I need for a hydronic baseboard heating system?"
Harris Woodward addresses the Phoenix tank heater in his Q&A "Is it worth it to pay for a Phoenix water heater or buy a condensing tankless water heater?"