Helo Maik!
I think in there is no direct way to change their passwords not voluntary as Initial for this type of user (dialog), only edit the table usr* manually. It is security feature. Why after 14.01.2015 you want to know all users passwords ? Don't you think that is too much knowledge for basis? 
What if someone says that it's you did something in SAP under his login because you know his password ? Maybe you should generate initial password (in excel for example) and paste it into SU10 transaction for all users ? All initial password will be unique but for one time.