CREATE TABLE #userTable (userid INT, logon VARCHAR(255));
CREATE TABLE #wFlo (x XML);

INSERT INTO #userTable VALUES ('1','abc@abc.com'),('2','bcd@abc.com');
INSERT INTO #wFlo VALUES (
   '<Parameters>
   <param name="id">8d878c4d-5376-4763-b27e-4c55e27270fc</param>
   <param name="sequence">1</param>
   <param name="logon">abc@abc.com</param>
   </Parameters>'
),
(
   '<Parameters>
   <param name="id">8d878c4d-5376-4763-b27e-4c55e27270fc</param>
   <param name="sequence">1</param>
   <param name="logon">bcd@abc.com</param>
   </Parameters>'
);

WITH a AS
(
   SELECT x,x.value('(//param[@name="logon"])[1]','varchar(255)') AS logon
   FROM #wFlo
)
UPDATE A
SET x.modify('insert <param name="userid">{sql:column("u.userid")}</param> as last into (/Parameters)[1]')
FROM A INNER JOIN #userTable u ON u.logon = a.logon;

SELECT * FROM #wFlo;

DROP table #userTable;
DROP table #wFlo