Is it possible - insert system tray expand to Sub-Dock ?

It used to be possible - until Microsoft made some core changes to Windows 11 22H2.

I'm using Windows 10 [1909], I tried dragging the tray into the sub-dock, but it didn't work, maybe I'm doing something wrong, maybe some other method is needed for this?

Ah, gotcha.

Since you mention sub-docks you are either running Nexus Ultimate (most likely) or Winstep Xtreme.

1. Open the Dock Properties dialog for the main dock. In the Systray section of the Content tab, check 'Show system tray in this dock' THEN 'Always show all system tray icons'. You can now uncheck the 'Show system tray in this dock' setting (I just realized that clearing the 'Show system tray in this dock' setting should not disable the remaining two options).

2. Create a new EMPTY sub-dock in the main dock by right clicking anywhere on the main dock -> Insert New Dock Item -> Sub-Dock

3. Open the newly created sub-dock, right click on it and in the context menu that pops up select 'Insert New Dock Item -> Show System Tray in this dock

This should give you a sub-dock showing all the system tray items in Windows 10.

Thanks, it's very simple, but I didn't know how