(This is using Inventor 6)
I believe you can do this by using "Parameters". I haven't had any experience with parameters in Inventor, but if you dimension a sketch and go under Tools->Parameters It will come up w/ a display like this.
From here I noticed that if you click the "Link" button you can open an Excel file/sheet.
Maybe you've already figured this out, but I can't really tell from your post. If you're looking for help with what to do with the Excel file I can't really help.