It’s not possible to reduce aggregate; you would need to evacuate data and recreate aggregate in new configuration.
It may be possible to create temporary aggregate using available spares, move current root, reconfigure root partitions and move root back. This is disruptive procedure and requires fair bit of experience. You may consider asking your supplier if they can implement this expansion.
You will still be left with active/passive configuration, with new controller working as pure hot spare. You would need additional drives to make it actually serve data.
By your description it sound's like a 7-mode system. as the other guy said before. you will need to destroy this AGGR0.
if you do that i would recommend you to re-install the system with Cluster mode (you need to convert the licenses, but it's free). this way you will not waste 3 disks for the other node root volume. you can benefit a feature called ADP that will save you that space.
I've configured new single chassis with similar situation. And I saw on my device just only Aggr0 with Vol0 inside. When I created new Aggr1 and tried to Copy or Mode Vol0 to new Aggr1 but failed. I cannot delete Aggr0. So how can I should to do?