Amelia,
if you are very, very careful you can do the following (assuming a 2D mesh, but more or less the same procedure in general in 3D): Check for the node numbers of the neighboring nodes, subtract 1 from each of the numbers as node numbering internally starts from 0. Save the model in ASCII mode and open it in a good text editor (e.g., notepad++). Look for the NODE section that contains the element definitions. Add a line for the new element with the three node numbers in counterclockwise direction to the section.
You will also have to give the element material properties etc., but this is better done using the user interface.
I would not recommend this procedure generally, but you may try at your own risk.
Good luck!
Peter