Of course they could because when the Confederate constitution was adopted Virginia was still a part of the United States. But to the point, South Carolina could still get slaves from Maryland because the Confederate constitution specifically protected slave imports from the United States. But conversely Maryland could not get slaves from South Carolina because Congress had passed laws prohibiting the importation of slaves from any source. (That assumes, for the sake of argument, that the Confederacy was a sovereign nation.)
Virginia did not need - this is just my schoolboy thinking - to join the Confederacy to get slaves from Maryland.
"But that is not the entire story in the context of Brother x’s question in post 54: “What could they (southern states) do in the Confederacy that they couldn’t do in the US?”" And I answered that - import slaves.
You must be forgetting that Lincoln actually did bring into the United States slaves from the Confederacy - not that it has any real connection to Brother x’s question.