Of course row can be added by addRow command
In case of treeGrid it has two additional params
grid.addRow(id,[data],index,parent_id,image);
parent_id - id of parent element ( 0 for top level ) , mandatory
image - path to image used for row, optional
the index can be set to null
To simplify row adding two more methods exists
grid.addRowAfter(…
grid.addRowBefore(…
Please beware that adding row by command is slower that adding rows by loadXML or loadXMLString in case of mass row adding.