It is not possible to import only the items via the documents import.
It is, generally, useless to import the items separately. According to the behavior specified using the field 74 of the documents import, it is possible to automatically create and/or update the items each time that a document is imported or updated. This behavior is generally satisfactory. In a third party tool you'll find all the items, while the WinBIZ database will contain only the items really used in WinBIZ. The third-party tool will become the master database, and the changes in the items will be transferred to WinBIZ only when new documents are added or old documents are updated.
In the case that there is the need to have, from the beginning, all the items of the third party tool also in WinBIZ, this is also possible. This first "migration" of the items must be performed from "Tools -> Import/Export -> Import Wizard". The process is documented in the WinBIZ help. The structure of the items table can help: Structure of commonly used tables