How to add a dm-cache to an already existing luks setup?

I've got two physical devices, a fast SSD and a large HDD. The SSD is in VG ssd and ends up serving as mountpoint /, the HDD is in VG dump and serves as mountpoint /home. Both go through LUKS for encryption, both are formatted as ext4. I could easily spare 40gb on the SSD to serve as a cache for the HDD and I'm wondering if I can change my LV-setup without reformatting.

Is it possible to reduce the size of the root filesystem, then reduce the size of the LV, add two new LVs for cache and metadata and attach the newly created cache-pool to serve the HDD? Remind me on how to setup in such a way, that unencrypted data does not leak to the cache...