Alternatively, you can do this without the Brenner interaction model but with the built-in minimization which uses the Universal Force Field (UFF) and the Add editor (shortcut: A) (it's Carbon by default).
First, you will need to modify SAMSON preferences (Ctrl+K):
-1- For the Add editor, disable the "Adjust hydrogens ..." option.
-2- For the Minimization, disable the "Warn me when hydrogens appear to be missing" warning to prevent it from poping-up on each new bond connection.
Then you can apply the minimization: Home >Simulate > Minimize. Start connecting atoms in the nanotube and the graphene sheet using the Add editor (A) - to create a bond simply connect two atoms with this editor.