Underscored line automatically down in colums depending on rows
 

I have a document that are automatically put together from other files. The
lenght of the document varies depending on the files it get the information
from.
The list i want to get here needs to have a underscored line during column
K, L and M. The thing is that i want to know how to put a underscored line in
all the columns and in as many rows in the document as there are information
in cell J!

How do i do? Is the question clear?