[CentOS] how to setup account which can 'su" to another account (NON-root)?

we have CENTOS 5.2 on DELL server.  we need allow a user can "su" to another user without password.

for example:

account user1 can "su - user2" without password.   (user2 is NOT root)

I know this is big security risk but ....  Anyone know how to do it?

Thanks.