You can accomplish what you want using the builtin CD-R recording functions of Windows XP (if that is your operating system).
A description of Windows XP CD recording feature can be found all over the Internet, but here's one link with a simple how-to:
CD burning in Windows XP
When you use the Windows XP CD recording feature, your CD-R(W) will be burnt as "multi-session" CD and you can add data to the CD at a later time until it gets filled up.
Every time you burn a CD as a multi-session, some space is used to "link" the multiple sessions, so if you fill the CD up little by little in this way you can't put the full 700 MegaBytes on it you would otherwise be able to.
You can also fill up a CD little by little with Nero by starting a multi-session disc the first time you burn it, and then continuing that multi-session when you want to add files.
Don't choose to finalize the disc until you are sure you don't want to add more files - once you do you won't be able to write to the CD anymore.