Kamil, here is what you need to do:

At the top of your new modifyusers.html, add these lines:

#set($userName = $user.getUserName())

#set($selectedUser = $user)

In your new modifyusers.xconf, change the page-action at the bottom to call "User.loadUserSearcher" instead of "User.loadUserProperties".

I also had to copy saveuser.html and saveuser.xconf.  In saveuser.xconf, you can delete the User.loadHits action.  I also had to change the form in modifyusers.html by making it <form action="saveusers.html"> and changing the save link to a submit button.

When you submit the form to the saveuser page, it still passes the username as a parameter - However, if the passed username does not match the currently logged in user, it will throw an exception.