Hi Tom,
WebP Express currently doesn’t support bulk conversion, as I do not see much need for it. It would be relatively easy to implement, though.
May I ask why you are interested in bulk conversion?
The way this plugin works currently is that images will be converted the first time it is needed. An upshot of this strategy is that it does not waste disk space on storing converted images that are not in use (many of the thumbnails generated by WordPress are probably never used)
To make something like a bulk conversion with this plugin, you could grab the whole site externally using wget. Make sure to limit the download rate, as it takes time to convert the images. This method however will not convert images that are only shown under some constraints (such as media queries)
