If I open the solution that I created for the cube, the partitions I added dynamicly will not appear in the solution and would be overwirtten, when I would deploy then changed cube.
Correct, the partitions won't appear in your local, on your HDD solution file, but they do exist.
How do you handle this? Do you create a new solution that is accessing the cube on the SSAS-DB? If yes, what is your solution, if someone accidently drops the cube or it gets currupted in anyway?
You can create a new solution or (usually not recommended) connect to the live/Prod DB on the server. To do the first, follow the instructions in this post http://www.sqlservercentral.com/Forums/Topic786526-147-1.aspx - Note, for 2012, it's now simply "Import from Server" so you can import MOLAP or datamining models.
to connect to the Live - use File|Open| Analysis Services Database, and then choose the server and DB to connect to. Believe you can also save a solution file once you've finished doing what you're doing.
Steve.