Hello,
Is it possible to marry a demon and if so, is there a ritual I can perform to do so?

Even if you did "marry" you could not exist on the same plane as them unless you were seriously advanced and have completed the Magnum Opus or were close to it. You should also read the page about Demon Lovers, but a similar thing applies there.

It is possible, do the standard ritual and then in the request write that you and your demon want to get married.

You can structure the ceremony as you like, but for it to be valid, you will have to do it in a Satan ritual.

Organise everything according to the couple's taste, with what you have available, in the end what is important is the couple.