How do I open MS Word and MS Excel documents with Google Docs as default?
Currently, you have one thing against the feasibility, and that is Google Docs (even offline mode) is designed to work with the document you uploaded to Google Docs cloud (Remember why Google wants you to put everything on the cloud?)

If you don't feel like going to Google Docs and start uploading files manually, you can use Cyberduck, to batch upload all your documents all at once, and then you can use Google Docs to access the documents.

Note that I put bold on "currently". Who knows if Google might release a true offline mode where it can read your documents off your local hard disk, and seamlessly upload it to Google cloud.