Corona team has stated many times, that they have no intention to bring an option to convert to legacy materials. Physical material is the new way to go and the sooner you will embrace it, the better it will be for you, however you can still use the old converter even if you're using Corona 7, or newer. Simply run older Corona installer, choose "unpack files" option and replace new converter files with the old ones. It's probably possible to use both, older and newer converters at the same time, but i can't advise you on that.