Yes, this is possible. This is done by modifying the ZFS properties of a ZFS dataset by the ZFS plugin.
Or you can do this by command line interface: See here: Setting User and Group Quotas on a ZFS File System
All the settings are stored in the ZFS file system itself, not in any configuration file of Openmediavault.
Is there a file where the settings are written in?
No, it is written to the ZFS file system meta data.
Can you tell me, how the plugin has to be modified?
I am not at home. So I can´t look. As I remember in the ZFS plugin there is an option to look into the file system properties. Some of them can be modified. Please look at the oracle dokumentation to find the correct property.
Or where the modifications have to be done in the cli?
All this can be done by the zfs <option> command.
Sorry for the simple questions, but I'm absolutely new in that.
No problem. You´re welcome. 
Thanks a lot for your advices. With a lot of courage i succeeded meanwhile in setting up a test user with a quota. I think I got it now. I still know old DOS times and was already scared at that time to let the computer implode with a wrong word 
I still know old DOS times and was already scared at that time to let the computer implode with a wrong word
The ZFS plugin of OMV has only a limited functionality, intended to manage basic ZFS tasks. More sophisticated ZFS tasks can be done in OMV too, but then one should be familiar with the usage of the commands zpool and zfs.
Can you tell me, how the plugin has to be modified?
Go to the ZFS Plugin -> Select a ZFS filesystem -> "Edit" from the menue
There you can edit the property "quota" or add a new property which is not in the list.
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