Good news! :)
Exporting and preserving .webp images will be available in in5 v3.8.12. Because this is a web-specific format, .webp images will get copied to the output regardless of whether Use Source Image is selected (unless the image is cropped, rotated, or has FX applied...then it will be re-rendered as a PNG).
There's no ETA on the release, but you can check when the new version of in5 has been released by doing one of the following: