DISCLAIMER: As always, you follow this at your own risk - keep good backups, I take no responsibility for you doing anything whatsoever - you’re a big boy/girl now, take responsibility for your own actions!

So you can’t use MigrationAssistant because you already have an account (Alice) on the new machine (New) and don’t have the disk space on the old one (Old) to turn FV off? Fear not! Let’s assume the username you want to move is steve:

1. As Alice, create (admin) account on New with same username, steve, and same password, and enable FileVault.
2. Replace /Users/steve/steve.sparsebundle on New with /Users/.steve/steve.sparsebundle from Old. (you could use scp, rsync, an external hard drive, or any other method to transfer these files) (I did this with steve logged in, but you can do it logged out though the path may be different)
3. As Alice, fix the permissions on New: chown -R steve /Users/steve
4. Log in as steve on New (unlocks the sparsebundle and mounts it at /Users/steve)
5. Fix permissions again: either as Alice or steve, sudo chown -R steve /Users/steve
6. Log out an in again Voila!

If this helps you, please drop me a comment :)