Yes, you can edit QuickSheet documents with MS Excel.
At least documents saved to Documents -folder (either in phone memory or memory card) are visible e.g. via PC Suite (Phone browser. Also from memory card if you connect your device in 'Mass storage' -mode). To check where last document is saved (or to save to different location) in Quicksheet application, open application, select 'Options'>'Saving options'>'Save as'.
You can also sync these files/folders using PC Suite sync. For 'What to sync..' 'setting (under 'General' -tab) check 'File and Folder items' and then under 'Files and Folders' -tab check the related settings.