I've read about this before; IIRC, it was in the β-carbolines appendix of TIHKaL for one. So, at it's very highest concentration found, 644ng/g harman would equate to having to extract a whole literal ton (yes, 1000kg) to obtain anything like a useful amount. The 50 µg/g of tetrahydro-β-carboline-3-carboxylic acid compounds would equate to 50mg/kg which, while rather better, is still an awful lot of raisins for not much product.
Now, it may be that there are ways of optimising the amounts of β-carbolines in your fruit, if you're a viticulturist. I simply don't know. I would add that other β-carbolines (and β-carboline-3-carboxylic acids) are found in blackcurrants (TIHKaL again) and, given the reports of blackcurrant cordial greatly potentiating psilocybin mushrooms, this may be a more 'fruitful' (yes!) place to be looking.
Codonopsis pilosula (Chinese: 党参; pinyin: dǎngshēn), also known as dang shen or poor man's ginseng, contains similar β-carbolines to those found in blackcurrants.
EDIT:
Also posted here “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