I have some experience working with thermoplastics and heatguns and the like (mostly for custom Bolteron/Kydex knife sheaths). I can tell you there is a learning curve and it in this case especially can be expensive. Also some plastics will not react/behave at all the way you want or expect when heat is applied to them. Some will turn black and bubble while others melt with haste almost like wax. Remember this has to go back into the aquarium. Specific plastics such as those used for single use water bottles after initial molding will be nontoxic until damaged by heat, uv exposure or simple age so this may be the case with this plastic. Maybe a call to Hagen support could help you find out MSDS info at least.
So if you have the time, patience, and disposable income it can't hurt to try. I in no way mean to rain on your parade, I love tinkering. Have burned up a few electric motors doing it....
Much safer would be to take the bits with you to the local hardware store and see what bits of PVC or CPVC would work to make those mods. A quick sanding and couple coats of spray paint and it will look great.
Thanks for the reply. I was kind of thinking this wouldn't be really possible but thought I would ask. I think I'll just get some acrylic pipe and try my hand at making lily pipes. I've contacted Hagen in the past with questions but they always return with an unhelpful answer meant to cover their butts.