How to splice 220 line to use for multiple uses
Don't try this (at home, or anywhere else for that matter)

You'd have to defeat the function of the transfer device to actually power the heater (unless you wanted to use your generator to power it without throwing the transfer device over to the generator position).  Furthermore, once you did that, the generator inlet would be live, with live prongs sitting there waiting to zap unsuspecting folks with 240V.

And if you're planning on doing this because you don't have some sort of transfer device (transfer switch, breaker interlock) installed, then your electrical installation deserves a red tag from the local inspector, as backpowering the grid is a great way to accidentally zap unsuspecting linemen who are out there trying to turn the lights back on.