Charnock spent time on loan at Leigh earlier this season.
The 21-year old has impressed for Saints’ reserve team.
“I’m very pleased we have been able to secure Lewis’ services,” Bulls coach Rohan Smith said.
“He has impressed for St Helens’ reserve team and has a lot of talent, which we will look to develop in the side.”
“He is a gifted youngster and we are looking forward to seeing him gel with the rest of the boys.”