Some diagonals can be converted, and in fact William Optics sell a nosepiece to convert their SCT diagonals to a standard type:
https://williamoptics.com/products/a...r-wo-diagonals
But as far as I know there's no standard thread for this type of accessory so the WO one will only work with their diagonals. No idea if there's one that will work with your Celestron diagonal.