Zotero allows you to insert an item manually to your library.

Here is an example on how to insert a journal paper manuelly to your library:

Miller, J. M. (2001). A framework for the multiple roles of librarians in problem-based learning. Medical Reference Services Quarterly, 20(3), 23-30.

• Click   the New Item icon on top of the middle pane and click Journal Article as below

A list of editabel fields will appear in the right pane

Click to the right of the field labels to edit manually, the - and + sign to the right of the author field can be used add or remove one author. Once you fill in the content of the journal paper, the fields look like.

The item in the middle pane look like,

You can simply add other types of publication items in your library in the same way.