[quote=@Darcs]
Few questions- How does creation of supply centers in one's own territory work?
Trade? I understand we can probably trade for territory-- but what about units? Can I trade for units?
[/quote]

You do not create supply centers, [s]supply centers create you[/s]. The only way to have more is to take somebody else's.

Yes you can trade positions and units so long as your/his supply centers allow it.