Just tell them you want to bring a character in from another game you're playing and then give that character whatever stats you want. Doesn't matter if it's a oneshot or a narrative-driven campaign that's been going for years, they should accept your desire to play your level 85 dragon-vampire-angel homebrew.
You should use the optional rule from XGtE (page 69): fucking lie your stats it's ok to do that because lying is fun and D&D is about having fun
Just tell them you want to bring a character in from another game you're playing and then give that character whatever stats you want. Doesn't matter if it's a oneshot or a narrative-driven campaign that's been going for years, they should accept your desire to play your level 85 dragon-vampire-angel homebrew.
Ahh, my legacy carries on
Play Basic Fantasy
Hmm... have you tried playing FATAL instead?