The Open XML SDK cannot work with *.bin files. It is possible to access these files, but they're in the old binary file format and thus only barely human-readable and difficult to code against.
What might make more sense would be to replace the custom toolbar with a Ribbon customization. You already have the VBA code, right? So you could adjust the macros to be used as "callbacks", create a Ribbon XML that calls the macros, then remove
the old custom toolbar. The old toolbars are still accessible via the Office object models, they're called CommandBars. So something like CommandBars("NAmeOfToolbar").Delete should work.
You should be able to get help with the above in the Excel for Developers forum.