Is it possible to make any customization? Is source code encoded?

No, it is not possible to make any customization, since the source code from version 5.x.x.x  is encoded.
However, you can still customize the design of user interface and you can customize or develop plugins.

Here you can find help for developers: http://support.qualityunit.com/432870-Developers-help
and programmers documentation: http://support.qualityunit.com/871709-Programmers-documentation

You can discuss adding new features with use.. just contact use via email with your request
and we will tell you if is possible to do this customization and what fee will charge for it