Well, if you just want to get the objects and connections into OmniGraffle, and aren't concerned about making the OG diagram identical to the wbstool diagram, you could export the file from wbstool in Microsoft Project format, open in OmniPlan (don't have to own it, there's a free demo for 14 days), export as OmniGraffle.
OmniGraffle stores its diagrams in XML representation, and wbstool can export as an XML representation. However, just because two different tools can both write out representations in XML does not mean that one can successfully read the other's files! It's more like an agreement that if a rectangle needs to be described, here's how we'll do it, but understanding that this rectangle represents a WBS block and that one represents a button on a wireframe isn't there in a form that the software can divine. On the other hand, a human can probably figure out what it all means, and with enough effort, write a converter of some sort, or just strip out the information needed with a text editor. To my mind, that's the prime benefit of storing data as XML.
As a helpful tip, postings to the forum aren't necessarily seen by anyone at Omni in a position to do something, so if you have a question where you need to get an answer (or want to make a feature request), you're better off using the Send Feedback facility in the app, or doing that in addition to posting on the forum. You still probably won't get an instantaneous answer, but you should get one sooner or later.