Old versions of Microsoft Word used WBK files for auto-saving documents and thus it should possible to recover them as save as DOCX. This might be possible only using the same version of Word that created the WBK file however.
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.
Articles and other information relating to wbk into docx conversion:
How to recover unsaved or corrupted Microsoft Office and OpenDocument documents - Salvaging data from broken DOC, DOCX, XLS, XLSX , PPT, PPTX, ODT, ODS and ODP files