Concerning mounting pulley to shaft

I have a pulley with an 8mm threaded bore:

I also have a turning SK3 motor. However, the shaft is not long enough to mount the pulley so they align, hence i used the supplied extension, which was also threaded:

The threaded length is not enough, and it leaves about 5mm of the threaded bore un-mounted!

The extension shaft is basically pushed onto the motor shaft, and the push fit allows it to move without any relative motion.

Now, under human load, the pulley “unscrews” off, and the extension shaft just loosens, so it cannot be push fitted anymore.

With this info in mind, how can I mount this pulley on this shaft?

so there´s no threads locking the main shaft with the extension one?

None, it is a push fit.

Okay, sry my english is not the best. Don´t think that will hold with any glue or a clamp or something like that. If you have access to a mill you can buy a bult with the same diameter. cut it to the right lenght. drill the none threaded side with a drill, and make new threads for the shaft… that will hold and should not cost you more than 1€

How thick is your motor mount ??