Yes, it will write just the bit selected and won't finalise the disc. What you're trying to do is to create what's known as a "multisession disc". A multisession disc enables you to burn data to the same disc in more than one writing session. If you have space after a writing session, you can write more files at a later date by using a multisession disc.
Nero 9 is my preferred method of creating multisession discs, but there's lots of free software on the internet that can do the job too. I've not used DVD Fab 5 personally.