i dont think so.
a password is supposed to be encrypted AT_ANY_CASE.
and passwords are NOT SUPPOSED TO BE DECLARED even to the administrator.
If you have the administrative rights, you can always access the objects owned by other user.
if you want to login as the other user and do some work as that user we have custom code that is equalent to 'su' in unix ( works only from a dba account to non dba acccount).
to start with you can always , capture the passcode ,
change it to temp_passcode ,
login as the user with temp_passcode,
do ur business,
logoff and reset the passcode to wahtever it is.
THE ORIGINAL PASSWORD IS NOT REVELAED AT ALL.

this link, shows how to do this step by step.
else
our 'su' proc does the same in one step.

http://orafaq.com/faqdbase.htm#BECOMEUSER