Like the "pin" button, there could be a "dock" button to move the window of a plugin into the MultiDock. This would be much easier than dragging it there. Or just right clicking the pin button for this would be nice. After all, docking is just another way of pinning a plugin window.

you can already click the fx icon in the top left and select dock in multidock 

Here is an autohotkey script to dock the currently focused window bound to the key combination Windows+d

#d::
  MouseClick,Left,17,17
  Send,{Down 8}{Enter}
  Return
 
It works with any window, including plug-ins, which can be docked

Thanks for both tips. But still, a single mouse click way would be nice.

swamptooth
you can already click the fx icon in the top left and select dock in multidock 

+1 works great in platinum.
You can also simply right click on the effect or the synth and choose "dock in multidock" from the menu