Theoretically it is possible however not officially supported at this time.
I can only give your the possible methods but can't make any promises it will work
Option 1: transplant the boot drive and all the drives of your current machine to the new one (Deck might not think it is the same device in which case you will have to reclaim it)
Option 2: install hexos on the new machine, claim your new server on Deck, you will have to bypass drive erasure in 1 of the following ways
when it asks you if its okay to delete your drives , hit yes and it will fail (happened to me and multiple other users but no promises it will work for you)
bypass erasing your drives by plugging in 2 usb sticks and letting hexos make a pool with that
keeping your HDDs unplugged and hoping for the best when you claim your server
creating a new pool with drives that werent part of your original pool
Once you bypass the drive erasure screen you go into truenas and migrate the pool from there.
This may or may not work.