All I had to do was ssh'ing onto the dreambox, going into /etc/enigma2/ and secure-copying (scp) bouquets.tv, bouquets.radio and all userbouquet.* files onto my laptop:
ssh root@dreambox cd /etc/enigma2/ scp bouquets.* userbouquet.* cweiske@bogo:dreambox-settings/
After that, I booted the new dreambox flash image and copied the files back:
cd dreambox-settings scp * root@dreambox:/etc/enigma2/ ssh root@dreambox killall enigma2
The killall command is to restart enigma2 - the service daemon will notice that enigma2 is away and restarts it again.