Question: I'm trying to split my application into different sub processes, each one doing a very specific thing. Main reason is stability and better memory utilization because i use a conservative garbage collector (boehm-weisser).

I don't want to do it one huge process that does it all. 

Unfortunately the subprocesses must display some windows so they will need NSApplication instances.

But i want them to appear as one application in Expose and leave the menubar of the main process when they became active.

Is there any way to do this?

Answer: 
I believe there's no way to do this (without **a lot of** hacking).

The only possibility which comes to my mind would be to have a dedicated UI process (which is the main Application) and do a lot of IPC to with the child processes. In short: splitting MVC over processes.
There is no way to do this. On Mac OS X, an application that has the key focus is also the main application and, thus, owns the main menu bar, is drawn as in focus, etc... The entire system is built around this notion, including Expose, the Dock, the App Picker, Launch Services, etc...

I'm also curious as to why you aren't using the collector built into the system. The entire system has been engineered to work well with this collector. It is a conservative, exactly scanning (except the stack), almost entirely non-blocking collector.