Yes, you can definitely make a cell in Excel convert a letter to a number. Here's how you can do it:

1. Select the cell where you want to enter the letter and convert it to a number.
2. Click on the "Format" menu in the top toolbar.
3. Select "Cells" from the dropdown menu.
4. In the "Format Cells" dialog box, click on the "Number" tab.
5. Select "Custom" from the Category list.
6. In the "Type" field, enter the following code:
   Formula:

   0;-0;;@ 

7. Click "OK" to close the dialog box.

Now, when you enter a letter in the cell, it will automatically convert it to a number. For example, if you enter "A" in the cell, it will convert to "8". If you enter "8FF", it will convert to "8".

Note that this method only works for single-letter entries. If you enter a string of letters and numbers, it will only convert the first letter to a number.