Though this thread is rather old I stumbled upon it a couple of times. Therefore I'd like to reply anyway.
Mirroring of multiple ports to a single destination port still (as of ROS 6.3rc1) does not seem to be supported. But take your initial question and situation and you see it would not make sense anyway. You are asking if it is possible to mirror several 1Gbps ports to a single 1Gpbs port. As soon as the accumulated bandwidth of the desired ports exceeds 1Gbps information will get lost.
Also keep in mind that a 1Gbps source port can have up to 2Gbps traffic (bi-directional). But you cannot transmit more than 1Gbps out of your mirror destination port. So there's also going to be some packet loss happening.
We're also using pmacct on a server to gather statistics about our customers bandwidth usage. As long as we don't have a router with 10G ports we're streaming our traffic through a switch with 10G ports on which our pmacct server listens.
In case we get our core insfrastructure migrated to MT I'll take a closer look at the RouterOS API. Maybe I'll find an easier way to get the information we need (traffic stats per port / per vlan), instead of using brute force with pmacct.