First, add this import to your ~/.xmonad/xmonad.hs:

 import qualified XMonad.Actions.FlexibleManipulate as Flex

Now set up the desired mouse binding, for example:

     , ((modMask x, button1), (\w -> focus w >> Flex.mouseWindow Flex.linear w))

• Flex.linear indicates that positions between the edges and the middle indicate a combination scale/position.
• Flex.discrete indicates that there are discrete pick regions. (The window is divided by thirds for each axis.)
• Flex.resize performs only a resize of the window, based on which quadrant the mouse is in.
• Flex.position is similar to the built-in mouseMoveWindow.

You can also write your own function for this parameter. It should take a value between 0 and 1 indicating position, and return a value indicating the corresponding position if plain Flex.linear was used.

For detailed instructions on editing your mouse bindings, see XMonad.Doc.Extending#Editing_mouse_bindings.