Hello, @pinheiropt

To have sub-directories structure on old WordPress installation while activating the Network feature, you just need to add the following code in your current theme’s functions.php file.

add_filter( 'allow_subdirectory_install',
    create_function( '', 'return true;' )
    );

That’s it. Now reload the Network page from Dashboard and you should see option to choose between sub-domain and sub-directory. Once you complete the Network activation process by adding the wp-config.php and htaccess code suggested on next page on Dashboard, you may remove the code from functions.php or even can change the theme template.

I hope this helps. Let me know how it goes.

Regards,
Alex