Even if you were able to backup a single specific restore point, you would not be able to use it to do a system restore with.
If you wanted to be able to keep more restore points, then you could use the tutorial below to set how much disk space you would like to be used to save restore points with by system protection as Corazon posted above. System Protection - Change Disk Space Usage
However, restore points are not always reliable or useful in some situations. Because of this, either manually backing up files you do not want to lose, or creating a system image
backup would be more reliable.
Hope this helps,
Shawn UPDATE: System Restore Points - Manually Extract Files and Folders