actually you can..
just create an alias... setup a forward to forward A@domain.com to B@domain.com... however dont setup the A@domain.com email account. this way, there will be no mail stored on A@domain.com, but when emails are sent to A@domain.com, they will still be sent to b@domain.com..
the forwarders can be setup in the cpanel(site admin panel link in member zone)
this is what i did for one of my accounts on horde.
on my other account, which is on smartmail, they have their own Alias option...
EDIT: yea, i agree with manbearpig.. i would rather use pop than forward