The 29-year old striker had a short stint back at Killie last season after returning to Rugby Park following his initial spell which ended with his 2006 move to Rangers.
"Hopefully," said Johnston. "We've been trying to do that for a wee while. I think Boydy's keen to stay as well. And somebody like that can only improve us.
"He's a top quality player. He's played at the highest level and scored goals at the highest level. There are not many players who can do that."