[Axis]

My bars have barely any knulring and they slip after a few hours of hard riding. Can more knurling be put on?

 

[sittingduck]

Unlikely, since I'm guessing it needs to be done in a lathe.
Maybe try a little loc-tite?

 

[Stoked]

new bars.

 

[motofire]

A good machine shop could do it. Or you could get some grip glue from a moto shop and be done in minutes.