You can use the current active node as a cluster member. You can then
install a clustered instance of SQL and migrate the databases. The final
step is to turn off the original instance. Possible, Yes. Recommended, NO.
This entire project sounds like a low-availability solution. You are
reusing equipment instead of purchasing a certified cluster configuration.
The one-NIC cluster is possible but falls into the "worst practices"
category. Not having a test platform, even a virtual one, is asking for
even more trouble.
Clustering is a hardware and systems tool to help achieve increased system
availability. Shortcutting the hardware and the procedures will result in
lower availability rather than higher availability.
Geoff N. Hiten
Senior Database Administrator
Microsoft SQL Server MVP