I am looking for a way to insert text into the background of a cell, so that I can still enter numbers on top of that text - similar to a watermark except for an individual
Type your text in a cell anywhere. Copy it and it will be saved on the clipboard. Insert a rectangular shape anywhere. Right click and choose "Send to back". This will make sure it will be at the background. Right click and "Format Shape". Do to tab "Fill" and click on "picture or texture fill". At the "insert from" choose "clipboard". Now whatever text you have copied onto your clipboard will be in the rectangular shape. Resize the shape to fit the cell(s) you desired. Adjust however you like for example remove the rectangular lines, add shadow, change font, remove background etc.