Jared,
If you have already created a shapefile from X,Y data then the only way you can add more points to it is in an editing session. You can use absolute x,y to add the point to exactly where you want it. You can then reverse engineer the coordinates to the feature table using calculate geometry by right clicking on the field name in the attribute table.
As far as I am aware you cannot just add extra rows to the attribute table as they have to be associated to a geometrical feature that has to be drawn.
Regards
Anthony