Easiest solution would probably be to just replace the outlet with a GFI.

Or you could start all over and find a new matching set that will fit the devices you have. Plates are not expensive.

you can change the outlet to either a GFCI or use a Decora style duplex outlet

If not, is there a way I can modify it to fit? I want to get a decorative wallplate outlet that is a set with my kitchen light switch, but they only have the GFCI style, and my kitchen has a (single) duplex outlet.