Switching user before SCP file transfer

Hello Jeroen!

I'm trying to implement scenario:
1. It needs to download a file from remote host via SCP
2. To get access to it, I need to connect to remote host under user A, then switch to user B like "sudo su - B", for
user A has no permission to read a file I need
and user B has no permission to connect to that remote host.

It looks like a quite common case, but I can't make it working with SSHJ for I don't see an opportunity to call "sudo su - B" in SCPEngine before a file transfer starts.

I was trying to switch user for a Session before SCPEngine is created, but it always leads to "net.schmizz.sshj.common.SSHRuntimeException: This session channel is all used up" for session is not allowed to be reused.

I hope there is a way to do this trick, could you please advise?