Hi James,
It is possible to add new "Lead Ratings" to the existing list (Cold, Warm, Hot). You would have to create a new screen and base it off the "LeadRatings" Table. It should look similar to the screenshot below (You can use whatever name you want for step 1, and you don't necessarily need to add this screen to a tab):
With your new screen created, locate it from the list (however it was labelled), then click the Screen Name to edit the screen. You can do your best to recreate the screenshot I attached below. Just make sure to add the "Lead Rating" Field to your screen and then add a Button. You must Click "Edit" for the Button Object, give it a name in the first step (I called mine "Save"), then click Next to step 2 of 3, and Add the action "Save All Sections". I have added a grid to mine, which you can see from my screenshot, you do not necessarily need this, but it would help you see your current list. Message me back here if you would like help with adding a grid.
Once you have created your screen, click the "Save and Close" button on the left of the screen, then hover your mouse over your new Screen, and click the "View" Button that will appear on the left. You should see a screen similar to my screenshot below, and have the ability to add a new Lead Rating. If you do not have a grid, simply reload your browser, go to your Edit Leads Screen, and when you click the dropdown menu for "Lead Ratings", your new record should be added to the list.
Please let me know if this helps, or if any of the steps I mentioned are not clear, and I will do my best to explain.
-Ben