I'm wondering if it is possible to embed a Microsoft Excel spreadsheet into an .INDD file?
I would like to provide a link within my InDesign file so they can click it and open an Excel spreadsheet.
I know it would be possible to do this in the final .PDF, but what about the original .INDD?
You have asked two different questions with two different answers.
InDesign treats placed graphics differently than placed text files or spreadsheets. Graphic files can be embedded, but other kinds of links cannot. So you cannot embed the spreadsheet file inside the InDesign file such that the raw .xlsx file can be extracted later, No.
However, you can produce a link to the spreadsheet file such that you can rightclick and choose Edit Original and have the file open in Excel.
To do so, go to Preferences > File Handling > Links: Create Links When Placing Text and Spreadsheet Files and check the checkbox.