Water heater can be added to any incoming water line, either on hot or cold.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.htmlUnder-counter water heater located near tub will supply hot water just like ordinary household water heater. But it will not reheat water that circulates inside the spa tub.
Otherwise, a spa heater is made with electric element that can work with 120V or 240V. Water from tub circulates through tank that holds element. Element must be fully immersed in water.
To retrofit spa tub with this heater would require cutting water line near pump, running both ends of water line into tank with element.
Then of course you would need to retrofit adjustable thermostat to control when element is on. And integrate element operation into air control switch at tub that can override thermostat so element is not heating when tub is not in use.
Unless there is a pre-packaged kit to retrofit heater, I think adding heater would be difficult.
If you need further help, I’m available over the phone at
https://www.6ya.com/expert/gene_9f0ef4df2f9897e7
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