ext3 can't be transformed to reiserfs without making a new filesystem, so all your data from that partition is lost. eg. you need some free space to shuffle the the data back and forth.
you need
Code:
apt-get install reiserfsprogs
and to create reiser filesystem:
Code:
mkreiserfs /dev/hdXX
#or
mkfs -t reiserfs /dev/hdXX
where /dev/hdXX is the partition where you want to make it.
(all your data from that partition is lost in the operation)
Partition can be mounted as any other partitions.
If you don't have enough space, but you have some space, you can do the migration in parts. eg. first get some free space to your ext3 partition, resize it smaller, move it to a later part of the disk, now make reiserfs partition, move some data there, and now you have some more free space in your ext3 partition, so now you can repeat the steps (but now resize the reiserfs partition instead of creating it).
Tools you probably need: parted (or qtparted), resize_reiserfs (part of the reiserfstools package) and ext2resize.
