Certain versions of Microsoft Word might actually still support formats from MS Works and even allow users to save documents to these formats, for example WPS. Which means using these versions of Word might actually be the most convenient way how to achieve docx to wps conversion. Some features exclusive to new version of Word (like formatting) may not be saved properly though or may not even support old Works format anymore.
For versions of Word prior 2003, you will need to install special plugin from Microsoft called Microsoft Works 6–9 File Converter that allows users to open, work and save Works documents. But first you would need to re-save your .docx document to .doc because the older versions didn't use DOCX back then.
If you do not have access to Microsoft Word, you may want to try to get old Microsoft Works installation CDs. These were commonly bundled with new laptops and computers back then, so you may even have one at home. But again you will need DOC files not DOCX.
Unfortunately, open source word processors like LibreOffice Writer or OpenOffice.org Writer do not support saving to WPS format and can only open it.Open DOCX file Open WPS file
One of the many office suite software products
One of the most popular word processor program from Microsoft Office suite