You can use nc (net cat) as a proxy for ssh.
So for your example, edit your ~/.ssh/config
file to look like this:
Host servB
ProxyCommand ssh -q servA nc servB 22
As long as nc is in your path you should now be able to ssh or scp directory to servB
If you don't have nc you can do it with ssh -W if your version is new enough (>= OpenSSH 5.4),
Host ServB
ProxyCommand ssh -W ServB:22 servA
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