PDN unfortunately doesn't exist for Mac, since it's based on Windows software. Try searching for art programs with a clone stamp tool.
What a clone stamp does is it repeatedly c/ps the pixels from one location onto another, maintaining x/y difference between your cursor and the anchor point. Say you set the anchor point to (0,0) and then click on (20, 20). Then you draw a line from there to (50, 50). A copy of the line of pixels from (0,0) to (30, 30) will be copied onto where you just drew. So look for a tool that does that. Good luck!