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.
Depends on what you need to create.
If they are circular holes then I suggest to use some padstack-type solution. Be aware that holes with a larger diameter than ~5-6mm are probably getting milled since the PCB manufacture does not have such large drill bits.
If for example you need to side-plate a large slot (milling slot) then you should mark this slot on an own-created documentation layer. Then contact your PCB bareboard manufacturer so they understand what needs to be done.
In the end, if you have a padstack, you can set the max min tolerance in the library and count on it showing up in the drill table. B-)