phuckd,
My server, Bluehost.com, says this:
“Addon Domain Maintenance
…
When you create an addon domain, it will also create a subdomain. This is necessary for the domain to be properly added to your account. The url for the addon domain will still be https://www.addon.com, not subdomain.domain.com.
…”
Have you tried before? I mean, in theory I should just redirect the addon domain to the subfolder. But theory does not fit reality most of the times :'(