from what i see online the bare base layer is a fancy stretchy fleece type material. it certainly was not designed to be used with a wet suit but there would be no reason you couldn't.
i think i would be asking.....what are you trying to achieve?
if your current suit is well fitted (snug with no loose areas or pockets)then adding an under layer may not be an option. in this case, as others have suggested, adding another layer of neprene on top may be the answer.
however, if you do have a suit that is not really fitted that well, and you have room to wear something under it, you can certainly do that. a thin base layer material whether it is fleece or merino wool, would take up the extra room in the suit creating a much better fit. a better fit means less water will wash in and out of the suit which will def keep you warmer. i used to wear socks inside my wet boots. they prevented blisters, made the boots easier to get on and off, and made a big difference in the warmth of the boots.
if you do try a base layer of some type, make sure it is slippery enough to allow you to get your wet suit on and off.
you would have to weigh the cost against the benefit. the bare two pieces for example, will cost about $250 canadian. you might be better to sell your current suit and put that money towards a suit that fits better, may be a bit thicker, and keeps you warm enough for your conditions.
let us know what suit you use now, what type of diving you do, and what are the temps like etc etc in order to get some recommendations. it may be you are using a suit that is just not optimal for your diving. you might need to get into a semi dry or even a dry suit.