If Zotero can't automatically capture citation information for an item you wish to add, you can still add them to your Zotero library manually.
In Zotero, click the New Item button (it looks like a green plus sign) and then select the appropriate reference type. Then, type or copy-and-paste the reference data into the appropriate fields (author's names, title, etc.).
Zotero will automatically save your new reference, and you can then add it to your collections or groups and cite it in bibliographies.