You can also extend a note without Melodyne. Split it within the clip, and use SONAR's timing tool to stretch the end. The shortcut is to grab the right edge of the clip while holding ctrl; the edge turns into a yellow line, then drag to the right. This gives you a preview which doesn't sound very good, but bouncing the clip to itself invokes the offline rendering algorithm, which is pretty much as good as it gets with current technology.
If you don't want to stretch the attack, split further into the note, do the above, and then crossfade some of the attack into the stretched section.