Doone,
Concrete will stick to the plaster of paris, so it could be done. However you do run the risk of the POP not drying at the same rate as the concrete, and your POP may either crack, break or rehydrate to the point that it will seem to never dry out again and sort of moosh. Even if you wait until the POP is dry it would quickly soak up the concrete. You could however wait until your POP is dry, then spray it with a varethane or shellac, that will create a more water resistant barrier, then brush on some bonding agent and put your concrete mix on it.
But if your POP mold is solid without adding that extra layer, and you are making a silicone mold from it you may not have to beef up the back of the POP form.
Happy casting,
Dena

Personally I've never added a layer of cement to POP but it sure seems like a bit of a risk at this point considering you've put $10 into POP already! :) As Dena is saying, the dryer, cured POP would absorb the moisture out of the wet cement and that wouldnt' work. Once the POP is past the "hot" stage and dried out already, you could spritz the back of the POP enough to dampen the POP and then add your cement layer to prevent the absorption and it would probably stick.

If you're going to do any more plaster molding, you might want to seek out some cheaper means than buying those expensive tiny containers from craft stores. Home Depot, and I think Lowe's has the cheaper form of POP in a huge bag for about the same price (used for stucco). Not great quality but it works for quicky projects. For your best value, seek out a pottery supply store and buy some pottery plaster #1 in which you'll get a huge bag for a little more than you pay for the smaller container at a craft store and far superior in strength and smoothness. Then there are "steps up" from that grade but you need to consult a supplier to find what's available to you.

I have done some leaves, one rather large, with the POP and with cheesecloth embedded . They're not of a great thickness but served the purpose of making a latex/silicone mold from. I still have those in an area of the garage where they won't break in case I want to make another silicone mold. I personally wouldn't put a lot of expense into those other than for the purpose of getting a couple of good latex/silicone molds from.

Ah....ok. Well, then I guess I'm ready to try a silicone mold.