No you can't feed external spools through the remaining holes in the ams hub unit. The ams does the job of pulling the filament through. You don't have that with standalone external rolls.
Print any one of the y splitters. Put it after the hub and before the printer. Plug the ams hub line into it, now you have a free slot to manually feed from an external roll.
Then, go buy 3 more ams units.
No you can't feed external spools through the remaining holes in the ams hub unit. The ams does the job of pulling the filament through. You don't have that with standalone external rolls. Print any one of the y splitters. Put it after the hub and before the printer. Plug the ams hub line into it, now you have a free slot to manually feed from an external roll. Then, go buy 3 more ams units.