I don't believe it is possible to add new classifications at this time. However, what you can do is create a new Library for your company and apply that to the new title blocks which you create. You would still have to classify them based on the existing classifications, but adding them to a specific Library will allow you to further filter which title blocks show up to the users.
To create a new library, go to the Library tab > Libraries Manager > New.
Also note that you can specify which libraries can be used for projects by going to the Project Configuration > Libraries and Palettes.
You can also choose which title blocks are used by default upon insertion of a new page.
I recommend you create a template that uses these configurations to set standards and defaults for your company if you have not already.
I hope this helps! Let me know if you have questions.
Thanks a lot for your answer! Your answer is quite detailed and it's nice to see when people understend what I am asking about... As for creating the new (mine) library I've done it at once but the information about specifying which libraries can be used for projects is very useful and satisfies my needs a lot. As for creating a project template I know this functionality and I understand how it can automatize disigning but if I hadn't known about this functionality the information you gave would be very important! So thanks and good luck!