Would I be okay to make my own hamster toy putting the sellotape on the inside?

Wouldn't it be a smarter idea to make the steps out of something that will not harm the hamster? Or that is going to hold up longer than cardboard steps (which he will chew through) would? Why not just a series of different size wooden blocks, glued together, to make the staircase? It would be safer and last a lot longer.

Yes hamsters like anything to play with

I made little ladder sort affairs for my rodents from Popsicle sticks. Just stuck together with Elmers white glue which is non toxic or chew hazard.
A lot of things at craft stores you can make safe toys and cage amenities from.
You could improvise a ramp sort of thing or even look to store bought. I have seen little climbing nets of sort at pet stores, even made similar from natural fiber rope. (no nylons or polly)
Really the only considerations would be chew hazards like plastics, choke hazards, or toxic things in the cage,
Super glue is non toxic after it cures. A lot of flexibility to get creative with.
Consider though, hamsters don't have quite the balance for climbing like their long tailed relatives.
Cello tape, even duct tape can be chewing hazards for rodents, I would avoid those.

Make your own hamster toy out of rat poison

cut slots into the cardboard and slide the pieces together, no cellophane. Or go to a local thrift store and buy a set up for it.

If he has already overcome the obstacle he doesn't need your help. If you do what you are proposing, he will most certainly chew on it and some will be ingested. Why would you ignore that fact?