i dont understand your requirement.
But if you are looking into somethin like a 'su' in unix,
here you go ( refer the sample session..see whehter it helps you).
I wrote  this stuff, so as to login as 'that' user without actually 
knowing his/her password (to maintaine the privacy and TO AVOID the pain of altering the passwords and intimating the users)

[b]-- i am logged in as myself[/b]

dbadmin@laugh_chipdb2 > show user
USER is "DBADMIN"

[b]-- i am switching as the user TEST[/b]

dbadmin@laugh_chipdb2 > exec s.u('TEST');

PL/SQL procedure successfully completed.

[b]-- now i have dynamically reset the password of user TEST to magvivek[/b]

dbadmin@laugh_chipdb2 > connect test/magvivek@laugh_chipdb2;
Connected.
dbadmin@laugh_chipdb2 > show user
USER is "TEST"
[b]
-- do your business.
-- now login back as dba, an i reset the password of user TEST to whatever it was before.
-- in this whole process, the original password is not exposed at alll..[/b]

dbadmin@laugh_chipdb2 > connect dbadmin@laugh_chipdb2;
Enter password: *********
Connected.
dbadmin@laugh_chipdb2 > show user
USER is "DBADMIN"
dbadmin@laugh_chipdb2 > exec s.r;

PL/SQL procedure successfully completed.
[b]
--
-- check...
-- first the temporary password
-- next with the original password 
[/b]
dbadmin@laugh_chipdb2 > connect test/magvivek@laugh_chipdb2;
ERROR:
ORA-01017: invalid username/password; logon denied

Warning: You are no longer connected to ORACLE.
dbadmin@laugh_chipdb2 > connect test/test@laugh_chipdb2;
Connected.