There's no possibility to extend 'Preview.app', without re-compilation. The source code is not public, so there's no way.
Give up with
Preview.app, as it has more drawbacks, than benefits: small number of formats, performance of Preview.app is surprisingly bad as well... I took decision for myself to move alternative way.
I highly recommend to try XnView (freeware) (I mean
XnView MP, which is the enhanced version of
XnView Classic). Probably it's not looking that user friendly in terms of Apple style after Preview.app, but it's easily compensated with tons of nice features, and extreme speed. Regarding your purposes - it has Batch Conversion conversion feature, plus supports more than 500 image formats and export to about 70 different file formats.