Large xmlFile

My situation. I have a large xmlPriceList which consists of individuel customer priceLists. I need to read, modify, add and save new and existing nodes to the xmlPriceList(as new customers are added and current customers prices are changed) I would like to do this without using XmlDocument and thereby load the entire xmlPriceList to memory. My ideá was to use XmlTextReader, but it´s a read-only? Is there anyway to use XmlTextReader to find the node and then use XmlDocument to modify the Xmlnode? How to do? Thomas Andersson