Concatenating secure and reused passwords

Would concatenating a secure (i.e. Diceware) password and other, much less random, passwords be a good idea? This would be for a password manager, and, since it would replace all my passwords, why not concatenate all of them into one and append a secure one?

Intuitively, it would be more secure since the password is longer. The disadvantage, of course, would be that I’d have to type more.

Are there any downsides I’m not aware of?