Assign Step Model to user defined Plated through Holes

If the mounting hole is a package or mechanical symbol then you can map a step model the same as you would for any other part.

If the mounting hole is a package or mechanical symbol then you can map a step model the same as you would for any other part.

Yes the mounting hole is a package but i do not have required Step Models for every Mounting hole. Tried 3dcontentcentral but did not find any useful model there.

So what model are you looking for? Holes by default are shown as holes. You may see a place bound shape on top which you can either hide (3d canvas - Setup - Preferences - Symbol Representation, just show only the step model) or open the filename.dra and go to Setup - Design Parameters - Design tab and uncheck auto create place bound). Then make sure you delete one if it's part of the symbol.

Actually looking for 3.3mm plated hole with 0.45mm wide pads around hole. 

So these are shown by default - no need for a step model. If yours doesn't show like this try the things I suggested earlier or maybe you can show a picture of what you see or attach the dra file.

Yes. Mounting hole is shown once i uncheck 3d canvas>Symbols>Respective Mounting Hole. But the problem i experience is that the Copper plating inside the mounting holes is shown thicker than it should be. As a result, the actual drill size of hole is not shown correctly. Can't find anything to control Copper thickness in pad file.

As an observation, Copper plating thickness is set to 20 percent of hole diameter.