For instance, let's say I want to ask the question "I want to make an Anima: Beyond Fantasy die in the Rolisteam dice roller, how do I do that?". (As it happens, I do want to ask this question, since making an Anima: Beyond Fantasy die is nontrivial (explosions, for one, require specific behavior that I could not immediately figure out how to implement).) Is asking this question on-topic for this site?
Yes, questions about using RPG tools to simulate dice rolls for existing RPGs is certainly on-topic.
Questions about dice as they relate to RPGs are generally on-topic, as long as they're not some sort of open-ended request for help designing a game. Questions about RPG tools, as long as they are clear and properly scoped, are also welcome on RPG.SE. So narrowing this down, asking about using a specific RPG tool to simulate dice used in a specific RPG is definitely fine.
In particular, your example for using a dice roller tool to simulate a particular sort of die roll used in Anima: Beyond Fantasy seems very much on-topic.