[Exim] aliases

If I have a system user named johnny

Johnny recieves alot of spam addressed to johnny@???
I would like to bouce all mail delivered to johnny@??? but deliver
all mail addressed to john@??? as well as johnny@???.

According to the documentation this can be accomplished with these 2 lines
in /etc/aliases

johnny@???: :unknown:
john: johnny

I would also like to know if it is possible to have mail that is returned
(responded to with an undeliverable error) in this manner to be delivered
anyway.

I am running exim 3.35 on debian/stable
This is the default debian compilation available via apt.

Is there a rough approximation of the sendmail virtusertable I must use or
do I just have the syntax wrong?