Old versions of Microsoft Word used .wbk files for auto-saving documents for backup purpose and thus it should possible to recover them as save as original .doc files. This may 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 doc format.Open WBK file Open DOC 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