Is it posibble to dynamically add additional fields in a workitem?
One answer
No, this is not possible. You have to configure the attributes in the process configuration. This is a great article about what you can do: Customization of Work Items in Rational Team Concert
Comments
So the workitem customization mentioned in the article are the only things that I can do to customize RTC?
No, there is more you can do e.g. write your own presentations, add behavior and the like. You can also do some automation with Java. See https://rsjazz.wordpress.com/interesting-links/ for a collection of links to things you can do and also examples on the site for various automation and behavioral control.
There always is more you could do, often the question is if you should.
For example dynamically create attributes. Since the UI does it, it is certainly possible to do this with some API tricks, but there is no documented way to and you certainly also should not try to do it for various reasons.