Putting the THC production gene into a very closely related plant like hops (Cannabacae, Humulus lupulus) would probably not be too much of a challenge.
Can you imagine getting stoned off a nice IPA? Mmmm.
OR growing cannabis that grows as a 20-foot tall vine with dangling buds all over it?

But to make humans produce it would be next to impossible. We simply don't have any of the accompanying life-processes required. See, hops would be easy because it is a) related and b) already has resin glands. So inserting the genes to make those resin glands produce cannabinoid compounds rather than luplulins would be easy.