There are two definitions of Bal Tosif:
1 - Do the mitzva with a change in its form (for example: getting a 5th species along with the Four Species)
2 - Do the mitzva with intention when it's not its time (for example: Doing Netilat Lulav after Sukot with intention of fulfilling the mitzva of the Four Species).
My question is: If someone does Netilat Lulav with a 5th species AFTER Sukkot (combining the two definitions above), does one transgress the issur of Bal Tosif?
Please use sources.
I am posting my initial thoughts on the subject as an answer.