Changing the Username xpath attribute

[jojospecial]

Hello. I want to use as an username the item marked in bold (9999999991) in the CAS response I get from the server:

Array ( [edupersonorgunitdn:businesscategory:extended] => Array ( [0] => ou=test,ou=ach,ou=units,dc=sch,dc=gr;ΕΚΠΑΙΔΕΥΤΙΚΗ ΜΟΝΑΔΑ - ΕΜ [1] => ou=test,ou=ach,ou=units,dc=sch,dc=gr;ΔΟΚΙΜΑΣΤΙΚΗ ΜΟΝΑΔΑ [2] => ou=test,ou=ach,ou=units,dc=sch,dc=gr;ΔΗΜΟΣΙΑ [3] => ou=ach,ou=units,dc=sch,dc=gr;ΝΟΜΟΣ )

[edupersonorgunitdn:gsnunitcode:extended] => ou=test,ou=ach,ou=units,dc=sch,dc=gr;9999999991

[uid] => testgym
[mail] => gym@test.ach.sch.gr

[businesscategory] => Array ( [0] => ΔΕΥΤΕΡΟΒΑΘΜΙΑ [1] => ΔΟΚΙΜΑΣΤΙΚΗ ΜΟΝΑΔΑ [2] => ΕΚΠΑΙΔΕΥΤΙΚΗ ΜΟΝΑΔΑ - ΕΜ )

[ou] => ΔΟΚΙΜΑΣΤΙΚΟΣ ΛΟΓΑΡΙΑΣΜΟΣ ΑΧΑΙΑΣ
[cn] => ΔΟΚΙΜΑΣΤΙΚΟ ΓΥΜΝΑΣΙΟ
[sn] => ΔΟΚΙΜΑΣΤΙΚΟ ΓΥΜΝΑΣΙΟ
[l] => ou=test,ou=ach,ou=units,dc=sch,dc=gr
[title] => ΓΥΜΝΑΣΙΟ - ΓΥΜ

[umdobject] => Account [edupersonorgunitdn:description:extended] =>
Array ( [0] => ou=test,ou=ach,ou=units,dc=sch,dc=gr;ΔΟΚΙΜΑΣΤΙΚΟΣ ΛΟΓΑΡΙΑΣΜΟΣ ΑΧΑΙΑΣ
[1] => ou=ach,ou=units,dc=sch,dc=gr;Αχαϊα )
[samlAuthenticationStatementAuthMethod] => urn:oasis:names:tc:SAML:1.0:am:password
[authenticationMethod] => gr.uoa.devel.cas.adaptors.ldap.BindLdapAuthenticationHandler )

I cannot understand how I can set it instead of "string(cas:uid)" that is already set in the Username xpath * field of the CAS plugin.

Any help would be much appreciated as I am in a production run of a platform that has to be used by schools and they cannot login using the default username, a think I did not know when I asked them to login and take action.

Thank you.