Adding .aspx page to existing web site

Hello,

I have a running ASP.NET web site. There is following problem with one of the forms:

One part of submitted data is added to database after "submit" is clicked. (to "table1")

Another part is lost (not added to "table2", and no files uploaded)



I have no source code of that project.
So, because I cant edit existing page, I came to a decision to add new .aspx page to a running application with same functions as existing one.

How can I do that? Or maybe you can recommend some other solution.

Thanks, Alex. 
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