Adding node to SQL 2008 cluster does not require SQL restart.
SQL Server Escalation Services
We had this question about SQL 2008 virtual cluster.
Question: Is it possible to add node in existing SQL Server 2008 or 2008 R2 Failover cluster instance without downtime (without stopping SQL Server or MSCS services)?
Answer: Absolutely! SQL Server 2008 was designed with rolling installation and upgrades in mind. To confirm:
I built a 2-node Windows 2008 R2 Cluster using Windows 2008 HyperV
I installed SQL 2008 R2 to the 1st node of the cluster
I started a long running SQL transaction
Next I installed SQL to the 2nd node and selected to join the existing virtual SQL Server 2008