The real question here isn't whether or not you can make an HTML form post to Excel.
The real question is whether or not any external control can target Excel and I suspect the answer is yes (though I have happily been Windows free for four years so I am proud to say that I have no clue about the answer to this question).
If you can get Visual Basic to speak to Excel or if Excel has some sort of API where you can pass in commands like this: "Row 10, Column 4 = 123", then you're half way there.
The next step would be to get the data from your form (in a web browser, I presume) into the hands of Visual Basic. If they can speak directly to eachother, great. If not, you might need to write data to a flatfile (web form writes, VB reads it, deletes it, and sends it to Excel). Or maybe a database would be a commonly shared point between the form and VB.
So there's your model. As for that minor issue of getting it to work, all I can suggest is a good book that specifically addresses an Excel API and communication methods between applications. I suspect VB might be that glue but I don't know.