I imported a dxf file into PCB designer to serve as the board outline. However; I cannot figure out how to convert the circles into plated holes. What is the easiest method to do this?
I don't think you want to do it that way.
The recommendation is to make a 1-pin component on the schematic (CIS/Concept level) that gives you the option of connecting that plated hole to your ground net, and as a component, already has the padstack and keepouts set up. As a bonus, you can place and reference this component by Refdes in your EMN/IDF exchange file.
We like being able to give the EE's CIS library descriptions like "plated mounting hole for M2 fastner with this size of washer" to place on the schematic.
Virtually eliminated those mistakes as the mechanical definition evolves.