Found this on a search....
You can dissolve propylene glycol in ethanol, specifically, said they are fully miscible, which means they mix together readily. and if you extract the thc to the alcohol, a la green dragon, then mix with propylene glycol, then boil the alcohol off, which is easier than it seems seeing as ethanol boils at a much lower temperature than propylene glycol. then yes, you could put thc in there.
and another
Tincture...is just alcohol (the kind you get at a bar) with thc disolved in it...so theoretically you should just be able to refill the e-cig cartridge with some tincture...and you're good. The only potential problem is...the atomizer is set at a specific temp...so if the propylene glycol/nicotine solution burns at a lower temp then tincture...it wouldn't work. BUT since propylene glycol is a form of alcohol, and thc disolves in alcohol, maybe you could make a tincture using propylene glycol, rather than regular alcohol that you'd drink.