Rehash Drupal 7 passwords into SHA1 I need to sync my user database with another user database; the other user database uses SHA1 as hash with USERNAME:PASSWORD as algorithm.
Is there a way to rehash it that I can store it in the database? 

Do you have any idea? 

It should be possible to edit the user/*/edit form and hash the password on my own and save it afterwards, or am I wrong? If it can work, can you give me any tip on how? 

I did a bit reasearch about editing the user_profile_form (for example: Making a username/password syncronization scheme - D7) where I read that $form_state['values']['pass'] could give me the password in plain text to hash it on my own and insert it into my database. 
Afterwards I directly tried to investigate more and tried to displa me the password with dpm($form_state['values']['pass']);, but I noticed (controlled with dpm($form, '$form info for $form_id '.$form_id);) that theres no values. 

How can I get the plain text password in user_profile_form or is it impossible and only possible in user_register_form (as stated in topic above). 
In my user_registe_form is no password entered, because I use the method to get a link with E-Mail were you can login one time and change the password with user_profile_form. 

Thank you!
There are a couple of ways to do this.

You need to look into the function user_authenticate.

In that function, you will see there is a call to variable_set() that sets the path to the variable password_inc. So, basically you could overide the path of this file, change it so it points to your custom password.inc, in a custom module out of core code.  In that file you would redefine the same functions with your own logic. 

There could well be a simpler way of doing this.  I know single-sign on can also be easily done with Drupal 7 as there are functions that make this easily available out of the box. Maybe look at OpenID code or other contrib modules if you choose to use that path.

Good-luck!