Multisite for Drupal 7: Subdirectory install for add-on domains

Using Drupal 7 in a shared hosting environment where I can host multiple domains.

I want to set-up several sites like the following:

www.example.com =>> main domain

add-on domains and subsites:
www.example.com/subsite
www.example2.com
www.example2.com/subsite
www.example3.com
www.example3.com/subsite

I have no trouble installing the main domain and the add-on domains(example2.com & example3.com).
I can't figure out how to install the add-on domain subsites in their subdirectories.