I am writing an application that supports custom fields. Currently I store all custom fields in a XML formated text field ( e.g. '<root><field1>val1</field1><field2>val2</field2></root>' in cust_field)

I am able to to use updateXML(cust_field, '/root/field1', '<field1>new value</field1') to update those values, however if I use updateXML(cust_field, '/root/field3', '<field3>new value</field3>') then it does not work since field3 is not in the old value. Is there a way to let MySQL automatically insert the new field3 node and its value into cust_field? I am thinking about stored procedure or even stored function but not familiar with both, can anyone point me to the right direction?