MGNLFORM-87 was released with 1.3 but there is still some work to be done.

Is it possible for users to write their own paragraphs to customize the critera logic?

From what i could tell its not. One way this could be implemented is if the form paragraph on submission scans its child paragraphs looking for paragraphs that have, in their definitions a nodeData like 'transitionCritera' that holds the class name of a custom class that implements just this test. What do you think?