Old versions of Microsoft Word used .wbk files for auto-saving documents and thus it should possible to recover them and at least save the text as a .txt file. However, this is only doable in the version of Word that generated the .wbk file in the first place.
Alternatively Corel WordPerfect word processor also uses the .wbk files extension, but for its default document format. So saving the text in a simple .txt form should be just a matter of using File ► Save as.. function. But that's different wbk to txt conversion, just sharing same principle.