Serotonin cannot cross the Blood/Brain barrier, so even if burning 5-HTP did cause it to metamorphose into serotonin, it wouldn't have any psychoactive effect.
5-HTP may be psychoactive in and of itself, but honestly, it seems like kind of a waste: bottles of the stuff are expensive and if you want to smoke something to get high, there are way more efficient methods.
Blessings
~ND