I have found this at a forum call DotSIS and it is a useful tool. It allows you to sign all unsigned applications. You do not need to use the computer to sign applications or games. You can sign the applications straight away on your phone with a free clicks.

You just need this software and you must have a Key and Cert file. You can now sign the unsign applications developed by talented developers. This is the fastest way to sign a application after you download the sis file to the phone.

Download MobileSigner