There are many formulas to add a comma after the first word in a cell. In case you have a list arranged across a column, you can replicate the formula across the list using the Fill option.

Sometimes it's not possible to separate data yourself before putting it into an Excel spreadsheet ... contains numbers. The default setting uses U.S. standards, but you can use commas to separate ...

If your Excel cell or row ... specifies where the first number ends and the second one begins. For example, if the cell contains the data "123-456," and you want to insert "123" into one cell ...