Adding an attribute to all nodes matching an XPATH expression using Oracle XML DB

I caot come to a solution to this task: my goal is to pass a cursor to a PL/SQL procedure and get the results as an XMLType. The function dbms_xmlgen.getxmltype() makes this task straightforward

<ROWSET>
 <ROW>
  <FIRST_NAME>John</FIRST_NAME>
  <LAST_NAME>Goodman</LAST_NAME>
  <HIRE_DATE>22-JUN-2011</HIRE_DATE>
 </ROW>
</ROWSET>

Now I want to add the cursor column data type as an attribute to each corresponding XML element.

<ROWSET>
 <ROW>
  <FIRST_NAME type="VARCHAR2">John</FIRST_NAME>
  <LAST_NAME type="VARCHAR2">Goodman</LAST_NAME>
  <HIRE_DATE type="DATE">22-JUN-2011</HIRE_DATE>
 </ROW>
</ROWSET>

This could be done using dynamic SQL, so I can write a PL/SQL function to get an associative array mapping each column to the corresponding data type.

Supposing I have both the aforementioned associativa array and the XMLType, how can I apply a set of transformations using a XPATH expression such as

-- pseudocode ;)
func(myXMLType, '//FIRST_NAME', ?add attribute to the matching node?)

Any other approach to get the job done will be fine