Hey, no this will not work as you've described it.
With 2 drives Hexos will automatically create a mirrored pool, meaning your data will be stored on both drives at the same time and you will only have the capacity of 1 drive but you can lose 1 drive without losing data. You cannot create a Pool without parity with more then 1 drive.
A RaidZ Pool type cannot be changed after it was created, so you will be stuck with your mirrored pool and you cannot expand it.
If you then throw in your 4 drives from the old NAS, Hexos will create a new RaidZ1 pool, which has the capacity of the numbers of drives -1 (capacity of 3 drives in your case). You can then copy the data from your mirrored pool to your RaidZ1 pool.
If however, your mirrored pool does not have enough capacity, get yourself a 3. drive to create a RaidZ1 from the beginning, copy your data over and afterwards add your drives from your NAS to the pool or create a new pool. (expanding a pool can only be done with 1 drive at a time and takes some time).
Also make sure all your drives have the same capacity or else Hexos will not group them together.