WordPress MU

mpmu installations work's perfect. Now i want that some of my script can work together with the userid & passwords of wpmu.

Is there a way to decrypt the md5 password and how? Or can i make a change to the way wpmy stores passwords. So i change it and the takes another format to store passwords?

You can reverse a MD5 hash. Check out this site . But the reversing is from a database, and not all combinations are available. It is not a mathematical computation- just a lookup from different databases.
There are other ways to encrypt passwords, most of them need keys- and that will have to be stored somewhere.

I could be wrong, but for your script to work together with the passwords stored by wpmu I do not think it needs to decrypt them, right?

Maybe its easier to explain what you want to do maybe there is another way to achieve it.

There's no need to decrypt md5 passwords, and you can only do it in a reverse way as pumpkin pointed to. If you want to check login information, you use md5() function and compare the hashes. If you want to change a password, you just need to md5() the plain text part and then replace the old hash with the new hash in your database.