I am well aware of how to associate a program with an extension using Default Programs in Control Panel. I only bring that up, because this question will likely sound like that.
I have a program (it is difficult to tell you about it because it is proprietary and I doubt anyone has seen it -- it is called Amazing Charts -- and it is used as an Electronic Medical Record or EMR). There is an Imported Items section where you browse to files such as PDFs and import them to a patient's chart. When you go into a patient's Imported Items, you will see a list of PDFs. When you open one, you can either open it in its native format so it opens in Adobe or whatever product you are using, or you can open it directly in a window to the right which is part of AC. This is changed via preferences.
One can also import Word documents or pretty much any document. One such document is an Excel spreadsheet. Once imported, it is simple to open it in Excel. But, it would be helpful to open that particular Excel document within the program. Is there a setting within Excelt that will allow it to do that. Maybe the program it is in (Amazing Charts) has to be compatible. It does allow PDFs to open inside it.