Is it possible to add an usb stick in my CRS125-24G-1S-2HnD and share this to the cloud, as a own service like “dropbox”.
Not cloud, but you can use SMB to share it on local network like a file share.
https://wiki.mikrotik.com/wiki/Manual:IP/SMB
thanks, i know how to use it local but i’m looking for a cloud solution over the internet. Maybe a nice future for a next release 
You could always enable the VPN feature of the router and VPN in to your network then access the USB through SMB.