I have epoxied and milled or drilled out and made cast iron inserts and pressed or epoxied them in. If you only have a few I would recommend Devcon Aluminum metal filler as it shines like cast iron. If you used plastic steel it turns black and it is easy to see it's a fix. I have had bad luck with Belzona and it is spendy as heck. Recently I have filled voids in Cast Iron with Muggy weld alloy 5 solder. It melts with a propane torch. With all fillers you need to open the bottom of the hole wider by grinding it like a cone or bottom is wider then top to help anchor it in. Also you should use a heat gun or propane torch to sweat out any oil that has crept into the pours of the iron. I use a propane torch and after it cools wipe with white cloth and use brake cleaner or an arosol that gets cold as it evaporates. BE SURE IRON IS COOL or you may burn down your shop...lol.
If you mill or drill out the table and your going to use an insert, be sure to drill a small hole thru the table so the air can escape as long as it's not thru and into the ways. I have also mixed cast iron into epoxy and I never heard back from the machine owner. In my business, no news is good news.
Rich