Doing it properly is very much recommended, but it is possible to do it via .htaccess by redirecting to the webp if its supported. You just need to upload both file types for each image. This should be a last resort though if the usual ways are not possible via the picture tag for example.
I havent tried this on a Kirby site though, so you may have to tweak it a little.
Nice tip @Syd - personally I use XnConvert which is free and cross platform, if I have a lot of them to do since it handles a heck of lot more formats along with batch conversion. If i just have one or two to do, then I tend to use Krita which is also free and cross platform