I've never worked with ALS patients, but, I HAVE worked with stroke patients -- back when I was still working (around the time the dinosaurs were roaming the planet

).
I was a certified home health aide. I was taught by the local Visiting Nurse Association how to transfer a patient safely.
Even though the patient cannot control his/her legs, the stability of the legs because of the long bones, together with the bracing that you would provide, should allow you to do a safe transfer.
If the lift, as mentioned above, isn't feasible for you, you might want to give your local VNA a call and ask them to show you how to perform a safe transfer.
Take care. Bless you.
Barb