Currently there is no automatic way for Zotero to convert a reference list from a text document into Zotero. However, there are online citation "parsing" tools such as AnyStyle that can parse your manually created bibliographies.
AnyStyle to Zotero instructions:
- Copy-paste your bibliography into the AnyStyle parser
- Edit the parsed references as needed
- Export as a BibTeX file to your downloads folder
- From the downloads folder right click the downloaded .bib file and select "Open with"
- Select "Open with Zotero" - the citations will then be added to a new collection in Zotero
- Be sure to double check the citation items and make any necessary edits