Can I override the WatchList alert pop up and sign in a visitor? Also, can extra information be added to the WatchlLst alert to confirm identity, such as address and date of birth?

There is a WatchList option to disallow sign in.  If this box is un-checked, the operator can sign in the visitor on the WatchList.
Home>Configure System? System Lists>WatchLists

When you add the person to the WatchList, you can specify other information, like date of birth and address, in the Alert Message box.