Tried and tested techniques exist for converting harmaline to THH. It doesn't matter where the harmaline was from, Rue will do. Zinc metal and acetic acid readily convert harmaline to THH. It's an ongoing question as to how THH occurs in caapi brews and this has been discussed here within the past two or three weeks.
I'm not actually sure if prolonged boiling of purified harmaline alone produces much, if any, THH. It's not impossible that harmaline could disproportionate into harmine and THH. It's probably better if there are other compounds present, as we would find in a whole vine (or quite possibly rue seed) brew, that would greatly facilitate the conversion process.
Some people speculate that the material out of which the boiling vessel is made also contributes to the THH conversion. To me, this would imply that glassware would be the least desirable type of container for the process in the absence of added catalyst and/or reductant/hydrogen transfer agent.
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli