You can check this topic, Mike's reply will help you, he had created ubi volumes with ubimkvol command.
http://imxcommunity.org/group/imx28andimx28evk/forum/topics/imx28-nand-boot-no-init-found
create a ubi volume as <volume_name> that has size of 50MiB
$ ubimkvol /dev/ubi0 -N <volume_name> -s 50MiB
-m means rest of space
$ ubimkvol /dev/ubi0 -N <volume_name> -m
Also, attachment at this link may help you
http://imxcommunity.org/forum/topics/imx28-nand-boot?commentId=4103...
The following discussion contains a bunch of info on setting up NAND. It helped me getting NAND with UBIFS running.