compare:
cat raw_expression.txt | head
cat raw_expression.txt | awk -F"\t" '{if(NR==1) $1="NAME"FS$1}1' OFS="\t" | head
$1 is the first field for each line.
$1="NAME" FS $1 assigns NAME to the first field and seperate by a field seprator (FS), which is a tab.
1 is always TRUE, so awk prints out the rest of the lines.
then, add a second column with a dummy "na":