Pasting from docs:

If you want to customize any specific column properties, and let other columns be displayed from table definition, you can pass 2nd argument of set_columns($cols,true) to true.

$col = array();
$col["name"] = "company";
$col["edittype"] = "textarea";
$cols[] = $col;

$g->set_columns($cols,true);

Only column with name ‘company’ will be changed to textarea and rest table column will be displayed as they were before.