is it possible to enroll as a tester ?

If you want to become more active in the development of Jazz products. Discussing this with your sales/account team is a great first start. When a new major release is going to be published, there are some customers who can engage in testing the Beta before it is actual posted to jazz.net. There are weekly calls regarding defects, enhancements and usability of the product, where client opinion is what matters most. Your sales/account team can keep you in the loop about these potential offerings. This typically only happens during Major releases. As far as fixpacks and minor enhancements are concerned. Some products on Jazz also release a Beta version of these features. Installing and using the Beta version with participation within the RFE community, tied in with social business (many products now have a twitter or facebook) that will allow you to participate in discussions and make your opinion heard. 

As far as I am aware (anyone can correct me if I am wrong). There is no such role where a client is assigned x amount of hours per week to test products outside of what I have mentioned above. Could this potentially happen? Not sure, but again discussing this with your sales/account teams, AVP (if you have it) is the best place to start.

Hope this helps. Thanks!