Is not that complicated, in Maya render the Z pass using the render passes, create the Camera Depth render pass.
Then in Nuke you need to create a ZBlur node, your tree should have the depth layer with the Z channel (the Z render pass), and you select this layer in the Z option.
you have to adjust the focus plane until you see the green area (if you have selected the focal-plane setup option) and with depth-of-field option you can increase the green area.
And that is, it's all in-built so you don't need a plugin or anything. Let me know if you need more help.