What you are talking about is "volume spanning" and that is what you should look up. you will find links such as this http://www.fantomdrives.com/products/raid/what.php4
I think you may be able to accomplish this with a program such as Partition Magic 8, notice from the article that this is like RAID, but without any speed or redundancy benefits.
Windows 2000 server, server 2003, and XP Pro support Dynamic Volumes which will let you add drives to any given volume. DO NOT use it for anything you care about which is not being backed up, because it ties to a unique number embedded in the hard disk controller and every detail about the exact configuration. If you break it in any way, you are in serious trouble.