Old versions of Microsoft Word used .wbk files for auto-saving documents and thus it should possible to recover them and save them in DOCX format. This might be possible only using the same version of Word that created the WBK file however.
Note: Corel WordPerfect word processor also uses the WBK extension, but for its default document format. You can use the File ► Save as.. function to re-save WBK to DOCX format. This is a different wbk to docx conversion.Open WBK file Open DOCX file
Corel WordPerfect is word processor for Microsoft Windows
One of the most popular word processor program from Microsoft Office suite
A Mac version of the popular word processing application