Can I add a third node to a BES 5 Cluster?

[stillabikeguy]

I need to replace my Server 2003 nodes with server 2008 R2 nodes, and want to leave SQL alone. Can I add a third and fourth Standby node and then decommission the old ones?

I would rather not have to build all new with new SQL and use the transporter if I can just add/remove nodes...

Thanks for any input!
Mike