I am not real clear on what exactly it is you are trying to accomplish, but here goes.
If this is a company application used internally, the IT Admin can configure the Office application to save documents as Office 97-2003 compatible format by default. The 2003 interops generally work with office 2007. (Not sure about 2010 though) However it can be difficult to control if the application is used outside your company.
Regarding interop for multiple versions have a look at this link which appears to address your question:
http://social.msdn.microsoft.com/Forums/en-US/vsto/thread/2b8dd0b3-a0b4-429e-ba64-4de6fdb45b6d/[
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Regards
[Edit] Another link for you to look at:
http://msdn.microsoft.com/en-us/library/dww0e04a.aspx[
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