Adding Custom Metadata

I am new to calibre, but I was wondering if it were possible to add my own metadata. I would like the following to be added:

<meta name="fixed-layout" content="true"/>
<meta name="orientation-lock" content="landscape"/>

If that is not possible, I would like to know if there was a way to lock the orientation to landscape, since it is a children's picture book and the pictures are in landscape.

Thanks!