Can I add my own field to every company on the Companies tab?

Open this screen in Saral/accounts/company-structure?tab=companies

Yes. On Company Structure → Companies use 'Add your own column to this table' — it applies to every company in the group. The Active filter hides closed companies; All shows them too.

Not what you needed? Inside Saral, ask Kuku (Alt+K) — if it cannot answer, “Ask a human” sends the question to Saral support with the chat attached.