Data Validation > List > Default Entry?

I'm assuming you mean, "When I start filling out data in one row, I'd like Sam's name to appear in a particular cell, but have a data validation list I can use when I get to that cell so I can choose another name if needed."

First, remove the data validation from those cells.

Next, let's assume column A is the first field for all rows and this data validation is over in say...column D. In D2, enter this:
=IF(A2="","","Sam")

Now copy that formula down the data set as far as needed. For all the rows that have no data in them yet, this column will remain blank. As soon as you later add the first value in column A, "Sam" will appear.

Now, highlight the entire column/dataset again and reapply your data validation list to it. Now you're ready to go.