What you're asking about is how to "automate" Excel. That's too big a topic for a discussion thread but Microsoft has plenty of documentation on the subject if you care to read it. Basically what you will need to do is open Excel, load the document, plug your values into the appropriate cells and read the results from wherever the worksheet puts them. It would probably be easier, not to mention cleaner, to simply code the functions in VBA.
there is a perfect solution for you! With the GISconnector for Excel you can work as you desire. You can do all your calculations in Excel and then synchronize with ArcIGS: http://www.gis-connector.com.
It is much faster than programming with VBA.