There is no means of entering multiple URLs into the site blocking using the avastUI.
However, you could manually edit the webshield.ini file using Notepad and copy and past some into the [WebScanner] section of the webshield.ini file. each entry has to be separated by a ; and if you use wildcards their use has to be correct or you could be blocking sites that you didn't intend.
See image example, were I created two site blocking entries *google.com* and yahoo.com, just to show where they would go in the webshield.ini file.
Personally, that said, the site blocking really isn't designed for this multiple entry or blocking and you should use a better tool, HOSTS file (or HOSTS file manager), Firewall, Browser (add-ons, etc.), most of which are designed for this sort of thing.