Redirects from addon domain folder to new domain

I have a cPanel account with a main domain and 5 additional domains.

Problem: The folders of the additional domains are not named like "additionaldomain1.com" but something like "website1", which, I think is causing the additional domains sites to show when accessing maindomain.com/website1. This I know would be considered duplicate content by Google and I'm trying to optimize the whole account.
I've tried making a redirect but cPanel tool is not allowing me because it would make a redirect loop.

In other words, how can I get rid of / redirect maindomain.com/website1 to additionaldomain1.com ?

Thanks in advance!