Yes you can, with a bit of work.
First you have to unlock the background objects. There should be a tips topic on modifying the template at the top of the map describing the process, but essentially you right click the background of the diagram, Select Background > Unlock All Background Objects and then edit background objects as desired.
The elements of the onion shape which are essentially a range of differently-sized circles should now be individually selectable. You can add a completely new circle simply by selecting a circle from Shapes in Objects on the Insert ribbon tab, then use the cursor to pull the circle margins out to the right size.
Alternatively you can click on an existing circle closest to the size of the one you want to add, then copy and paste it into the background (preferably away from the existing diagram until you have finished editing it) and resize it.
You then need select a colour and level of transparency for the new circle and make whatever other changes you want to it. Move it back in line with the other circles, then right click and use the Bring forward and Send Backward commands so that it shows up in the appropriate position in the diagram.
Finally you can use the same approach to add a description for the new circle under the Legend. When you have finished don't forget to relock the background objects.