There is no public method for this, but you can extend dhtmlXGridObject with the following method (place the code below dhtmlxgrid.js file on the page):
dhtmlXGridObject.prototype.serializeSelected = function(){
var out = “”;
if (this._mathSerialization)
this._agetm=“getMathValue”;
else
this._agetm=“getValue”;
for(var i=0;i<this.selectedRows.length;i++){
out+=this._serializeRow(this.selectedRows[i])+"</"+this.xml.s_row+">";
}
return “”+out+"";
}