Is there a seperate Debian package to download and install it on an existing system?
In theory it is possible to install it seperately, but this is not supported.
How to install OpenMediaVault from a USB stick?
Download the ISO file and execute the following command: sudo dd if=xxx.iso of=/dev/sdX bs=4096
The installer is asking me to plug a USB stick with firmware files on it for my hardware, e.g. NIC. Where can i download them?
Please check the Debian documentation or wiki.
What is the default password for user 'root' and the Web UI administrator 'admin'?
The default password for these users is 'openmediavault'.
How do i reset the Web UI administrator password if i forget it and can't login anymore?
Use the omv-firstaid command line tool.