A tool by Rod Page, code on GitHub
This tool takes one or more DOIs and creates "cites work" entries for each item. Note that this tool only works if (a) there is a Wikidata item for the DOI, and (b) CrossRef has citation links for that DOI, and those works cited also have DOIs and have items in Wikidata.
The output from this tool is a list of Quickstatements that you can add to Wikidata.