Adding custom share menu items is already in my “todo” list. Just be patient for new things to come. Also see here: https://wordpress.org/support/topic/lightbox-inquiry-link/
Also additional customizations may be added in future versions – however since I do all this in my spare time besides my regular job and don’t get paid for this, things may take a while. Donations by the community help to get things done faster 😉
In addition: all buttons are used as SVG and PNG, depending on what the browser supports (see skins/xxx/controls.png and controls.svg). If you want to provide new buttons, just send the files here: https://arnowelzel.de/upload and send me an e-mail (see https://arnowelzel.de/en/about-me for my contact address).