Install a Network under a mapped domain

This is possible, and it actually should be very easy for you to do.

There are lots of ways that you could have this setup, depending on if your server is using cPanel, Plesk, etc. At its most basic, right now, your Apache virtual host points to your main web directory, maybe something like /var/www/, and you have your htaccess file routing the domain to a subdirectory of that. What you need to do is change your virtual host to point to the directory in which WordPress actually resides instead of its parent directory. You’ll want it to look something like this:

<VirtualHost *:80>
    ServerAdmin [email protected]
    DocumentRoot "/var/www/sample"
    ServerName sample.com
    ServerAlias *.sample.com
</VirtualHost>

How you do that will depend on your server’s software if applicable. If you don’t feel comfortable making changes to your server config, you should consult your web host and I’m sure they’d be happy to make this edit for you.

Cheers~