do you think if I added an engine with a reversed prop I could generate enough power to led lights for an afterburner look in the pack of the pulsejet????
Sure. As
@jpot1 mentioned it'll be better to power it off the battery (it'll be much more efficient), but I don't see why you couldn't if you wanted to. With that said, I imagine it'll be a good bit of trial and error to get working well. Here's how I would accomplish this:
You'll want to keep the propeller pointing forwards. If you're using a brushless motor, add a three-phase full bridge rectifier and a smoothing capacitor to it to get an unregulated supply. A brushed motor shouldn't need a rectifier but a reverse-polarity protection diode wouldn't hurt things (along with another smoothing capacitor). Play around with the kV of the motor and the pitch of the propeller to get a "good" voltage: greater than the LED voltage at minimum flight RPM, less than regulator's max voltage at max flight RPM. From there, something like a LM7805 or LM7812 (depending on LED voltage) should provide decent regulation for whatever LEDs you hook up to it - just make sure you're not passing the current capabilities of the voltage regulator, split the strings and use multiple regulators if you are.