If you have the editor on the column, you can get the editor and set its value after adding a row.
onAdd: function(index,row){
var ed = $(this).edatagrid('getEditor', {index:index,field:'Key'});
$(ed.target).textbox('setValue', 'U')
}
You can also set any row values before editing a row.
onBeforeEdit: function(index,row){
if (row.isNewRecord){
row.Key = 'U'
}
},