I am not sure of how your final iMovie project is going to be shown, what format or if it is to be uploaded to a website or you need to author a DVD disc.
Also, not sure what version of OS X you are running on your iMac, either.
If you need to render/author/burn a DVD disc that will play on any kind of DVD player, you will need some DVD authoring/burning software to make the video DVD.
Apple no longer includes DVD authoring/burning software any longer with newer Macs (iDVD).
The only reason I am mentioning this fact as there is one software title that has both DVD authoring/burning AND screen recording capabilites all rolled up into one software package and that is Roxio Toast Titanium.
I use older versions of OS X (10.6, 10.8 & 10.9), so I was able to purchase a newer version of Toast (Toast Titanium 12) to update my much older Toast 8 version that I was afraid wouldn't fully function with all the later versions of OS X that I have available on my iMac.
I found Toast 12 fairly cheap and it is quailfied to still function up to 10.9.
http://www.amazon.com/Roxio-Toast-Titanium-Mac-Version/dp/B00L64Q6SE/ref=sr_1_3? ie=UTF8&qid=1454989498&sr=8-3&keywords=r…
IF your iMac is running OS X 10.10 Yosemite or 10.11 El Capitàn, you will need a more recent version which maybe a little more expensive.
http://www.amazon.com/Roxio-RTOT14MLMBAM-Toast-14-Titanium/dp/B0123C60F6/ref=sr_ 1_1?ie=UTF8&qid=1454989498&sr=8-1&keywor…
Good Luck!