How to insert dashes in a column of numbers in Excel:

1. Select the column of numbers that you want to add dashes to.
2. Right-click on the selected cells and choose "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: 000-00-0000
5. Click "OK" to apply the formatting to the selected cells.

This format code tells Excel to display the numbers in the format of three digits, followed by a dash, then two digits, another dash, and finally four digits. So, for example, the number 333333333 will now be displayed as 333-33-3333.