Hi I've been looking to create new subclass, its my first time making a warlock subclass and was wondering if there was a way to add spells to the mystical arcanum spell lists, we have home-brewed a seventh level spell and wanted warlocks to have access to it, is there any way for this to be possible?

short answer: Not currently possible.

That's fine, just thought id check thanks for the response

Happy to help, sorry I couldn’t give better news.

Its cool, will just have to find a creative way to implement :)

You could use a feat and just let them not select a regular Mystic Arcanum for that level.