I have used a standard soldering iron to modify and fix 3D prints in the past. You have to keep some things in mind when doing so:
Don't overheat your material. PLA can take a soldering iron of about 230 °C before charring.
Don't use a tip with solder, or you will get grey deposits in the plastic.
Round tips or flat tips both work fine, but you want to not ...
Cutting materials with a 'hot knife' isn't anything new. There should be a fairly established toolset for working foam. For 3D prints, I prefer to add coatings rather than modify surfaces with a hand held tool after. I do run a drill bit through printed holes though.