Maynard has 12 months remaining on his current deal but Millen has been keen to tie down a man who has netted 38 times since his move from Crewe in 2008.
"I don't want to leave everything to the summer. We know what we want to do and that's what we're getting on with," Millen told BBC Bristol.
"Nicky's happy to be back playing again, fit and scoring goals and we're delighted to have him. There's discussions going on, like there is with a couple of players."