The Excel connector does not provide a way to do this. You can interrogate Excel however and work with the message boxes directly. This is tricky however.
I created an adapter for Excel (set the Path equal to path to Excel). Created a sample worksheet with a message box macro I can trigger. I started interrogation, triggered the message box and interrogated it. I adjusted the match rules so it would match any message box. I set the StartMethod to MonitorAll so that this adapter will catch all instances of Excel started after it.
For this adapter to do its job, Excel must not be running when the adapter is started. So, your project should start your Excel adapter first, once the adapter is running you can start your Excel connector and perform your normal functions. When the message box is created you can dismiss it.