To superimpose an "X" over a number in an Excel cell, you can follow these steps:

1. Select the cell that contains the number you want to add the "X" to.
2. Right-click on the cell and select "Format Cells" from the drop-down menu.
3. In the "Format Cells" dialog box, select the "Custom" category.
4. In the "Type" field, enter the following format code:
   Formula:

   0;"X";0 

5. Click "OK" to apply the format to the cell.

This format code will display the number in the cell if it is greater than zero, and display an "X" if the number is zero or less. The "X" will be superimposed over the number, making it somewhat visible.

Alternatively, you can also use conditional formatting to achieve this effect. Here's how:

1. Select the cell or range of cells you want to apply the formatting to.
2. Click on the "Conditional Formatting" button in the "Home" tab of the ribbon.
3. Select "New Rule" from the drop-down menu.
4. In the "New Formatting Rule" dialog box, select "Use a formula to determine which cells to format".
5. In the "Format values where this formula is true" field, enter the following formula:
   Formula:

   =A1<=0 

   (replace "A1" with the cell reference of the first cell in your selected range).
6. Click on the "Format" button to open the "Format Cells" dialog box.
7. In the "Font" tab, select the "Strikethrough" option and click "OK".
8. Click "OK" again to close the "New Formatting Rule" dialog box.

This will apply a strikethrough formatting to any cell in the selected range that contains a number less than or equal to zero, effectively superimposing an "X" over the number.