You can try pushing into France during a civil war, but know that you'll swiftly be gobbled up by the king once it's over.
I took Anjou, with a weak claim. I only needed a single province to usurp the duchy, since Anjou consists of only two provinces. Then I made the kingdom. So you'll need some money saved up. Of course, in this game I was RPing as a dynasty of noble knights who refused to take part in intrigues, and this complicated things. I could have fabricated a claim instead, or murdered a few people to be in line for the throne.
SIDENOTE: Alan of Brittany, the only listed Breton king in the game's history files, was granted the title of king by the Holy Roman Emperor in real life. Apparently it didn't succeed him.
Salomon of Brittany was also called king, but only because of his pompous manner, and was not a de jure king.