Is it possible to add data validation to existing table without overwriting existing value

So we have a report that is within a Excel table and for some time, its been growing in size(row count), but now that we are really starting utilize the data in exec reports, trying to update certain columns with data validation lists, so that users are not entering whatever they want.

So my questions are as follow:
#1 because the report already has close to 1000 rows, and for the most part everything within the one column we are focused on is accurate, so is it possible to add a data validation list to this column and apply it down the table without overwriting the rows value in that cell?

#2 Once we have all current rows updated to include this list, how can we go back thru and identify any rows that may not have a value that matches anything in the list?

Currently if i copy down the cell with the list, it does copy the list down to the next row, but the original value is replaced with whatever the selected value was in the above cell.

thanks again, the 2nd option was what i was looking for.. worked like a charm.