Sure, I can help you with that! You can use the function in Excel to add a comma between the last name and first name. Here are the steps:

1. Select the cell where you have the last name and first name together.
2. In the formula bar, type
   Formula:

   =CONCATENATE(A1,", ",B1) 

   where A1 is the cell with the last name and B1 is the cell with the first name.
3. Press Enter and you will see the last name and first name separated by a comma in the same cell.

Alternatively, you can use the ampersand (&) symbol instead of the function. Here's how:

1. Select the cell where you have the last name and first name together.
2. In the formula bar, type
   Formula:

   =A1&", "&B1 

   where A1 is the cell with the last name and B1 is the cell with the first name.
3. Press Enter and you will see the last name and first name separated by a comma in the same cell.