you want to add multiple ips ( aliases ) to a network interface using webmin's Net module. Adding them one by one is a slow process.
Modify webmin's Net module to allow you to copy/paste ip addresses in a textbox ( one per line ) and then click once and wait for it to do it's job.
Here's the patch for webmin 1.410, I know it's an old version but it should be easy to adapt for newer versions, let me know if you need any help.
Log into webmin as root/admin , go to network configuration, click on an interface in the interfaces section, click the "Add virtual interface" link and now you should see a text box where you can just insert each ip on a new line. then just click the "Create" button