It’s easy to convert windowskins from RMXP to RMVX and RMVX to RMVXA, but not really from RMXP to RMVXA. This utility (in the form of a script) lets you convert RMXP windowskins to RMVXA format. And while I can’t test this out yet, it has some positive reviews so is definitely worth sharing.
So, here is the link to the RGSS3 utility. Install it as you would any other script. Get the RMXP to RMVXA Windowskin Converter.