I have exported all of the parts that I have entered into Rebrickable as a .pbg file. I also have a .xml file from Rebrickable that is for BrickLink.
I was told that I could just import the document into LDraw and then I could build projects just using the parts that I already owned.
I have LDCad and MLCad installed from the LDraw AIOI.
Is there any way to import these files as an LDraw parts list?
I don't think you "import the document into LDraw" since I don't think there is an LDraw program; it's a library of all parts. CAD programs manipulate those parts.
I tried various CAD programs over the years and recently found that LDCad (which you say you recently installed) works for me. So what you want to do is to import your Rebrickable parts list into LDCad as a parts library. Then, indeed, it's much easier to play with LDCad when you don't have to paw through the entire LDraw library looking for parts; you just go through your subset of parts.
Two years ago when I found LDCad I wanted a parts library restricted to just the parts in a specific LEGO kit so I kept notes and posted my experience. I still use it.