Solved..
read
http://wiki.contribs.org/AddExtraHardDisk1.) Setup SME with option #sme nolvm noraid (to setup root at first HDD-disk)
2.) Create raid5 with mdadm #mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/hdx....
3.) make ext3 at raid5 with #mkfs.ext3 /dev/md0
4.) create quota on md0-Raid5 #quotacheck -vugc /dev/md0
5.) mount new home (/mnt/home )and copy old home-files to new home at raid5
6.) modify fstab for raid5 as new home
7.) reboot
..ready
please correct me if i forgot something here..