Adding files to already burned DVD

If I burn a small file to a 4.7G DVD-R on my iMac, why can't I return at a later date and add more tracks? Seems like a huge waste of space. Is there aftermarket software available that will allow me to do this?

You need to create a Multisession disk. You should be able to do it with Disk Utility, which is on your Mac. This tells you how to do it:
OS X: How To Burn A Multisession CD (or DVD) | a Tech-Recipes Tutorial

Thank you, I never knew that capability existed in my iMac. I'll give it a try!