Hawley, who has penned a two-year contract with the Gulls, was a free agent following his departure from Scunthorpe, while Derby striker Ball has joined on a two-month loan deal.
Knill told the club's official website: "I've signed Karl Hawley and Callum Ball, which gives us four strikers and I'm delighted to have them.
"I know Karl really well from when I had him at Scunthorpe and he has always scored goals in his career.
"We've been looking for a different type of player to play with Elliot (Benyon) and Ash (Yeoman). We were let down on some but, all of a sudden, Callum has become available.
"I didn't think we'd have a chance of signing him but, fortunately for us, Derby County manager Nigel Clough has allowed him to move.
"Callum feels it will be a good move for himself and we're delighted to have players of this calibre come to Torquay United. Callum is initially here on a youth loan and we will look at it again in October."