New2tubntubby

I've purchased a inflatable hot tub spa, one with the separate pump/heater combo that attaches to the spa. My question is, can you add an additional heater that heats the water before it gets to the spa pump/heater. I'm trying to think of a way to have both heat and bubbles at the same time. My assumption is that all one would need to do is extend the water/lines and connect the additional heater off the spa then run that line to the spa pump/heater. It seems straight forward to me but I must assume I am missing something important. Any tips or directions to helpful info would be very appreciated.