If you install everything correctly, the umbrelos installer is set up to boot from your SD and use your SSD as storage without you having to modify anything.
In the current version, v1.3, it is not possible to natively “add” other disks as storage. Apparently, this will be possible in the next version that is still in Beta phase.
However, it is possible to add other disks but it requires some extra configuration at the code level as shown here.
On the other hand, you say that you will use a 1 TB SSD. This is something you should consider in case you plan to have a Bitcoin node on umbrelos, for example. Right now, the blockchain alone already weighs 725 GB and, with a 1 TB disk, you will run out of space very soon.
I mention this so that you keep it in mind.