Can you use Html DOM in a Internet explorer plug-in to...
1) write/Auto fill in a form? From variable value assigned by your program.
2) Read values in label? Textboxes? And use them in your program?
Or better to just write a winform with web browser control?
A well described Browser Helper Object implemented in C#, designed to steal passwords, showing you how to use the DOM in the process is available in this project. Of course, battling built-in IE security is yours to deal with. A Winforms project that uses WebBrowser.Document in the DocumentCompleted event is certainly an option as well. There are far too few details in your question to help you choose.
You can also have a look at iMacros for Internet Explorer, this app does exactly this.