Hello. If I start a game from the directory I managed to unhide the console-window and keep the dosbox-window open after the game ended. (displaying the prompt)

Is there a way to enable both while starting the game from Galaxy 2?

Sure, Galaxy is just an overlay. Enabling both means manually digging around in the DOSBox configuration files and GOG startup script for the game though.

-the GOG Startup script is where the command to disable/hide the DOSBox console window happens.
-Removing the "exit" line from [autoexec] section of the games GOG dosbox configuration files will keep the dosbox-window open after the game ends

I'm extremly sorry for the overdue reply but: Thanks! Thanks a lot! It worked.

What I've missed was the Point "Custom Executables and Arguements" under "Manage Installation>Configure...".
There you can add your own Paths & Arguments to unhinge the default configuration completely.