I want to know if I can add my own buttons to the UI interface to implement my own functions? 
For example, I wish users could have more image editing options(create filters for images or else) after uploading their own photos, so can I develop these functions into FPD on my own?

To get proper guidelines, you can open a support ticket. Thanks

Submit a Ticket: 
https://support.fancyproductdesigner.com/support/tickets/new