Well as you discovered, that’s one reason not to use excel.
- In excel save your original fcsv in csv format using save as.
- Open the csv file in notepad (or a simple text editor) and paste these on top of the file (including the # sign)
- save and try opening in Slicer now.
# Markups fiducial file version = 4.13 # CoordinateSystem = LPS # columns = id,x,y,z,ow,ox,oy,oz,vis,sel,lock,label,desc,associatedNodeID