If I understand correctly. It can easily be done.Make a block (the mould material) larger than the concrete block. Place the concrete block with the top side (the side that will be the open side of the mould) on a face of the mould block. Remove the face covering where the concrete will be and make sure the there are faces on top of the parts that will create center voids. Orient faces so all "FRONT FACES" are facing outward (contextual menu "orient faces"). Please see attached with the mould in transparent material (which I hope makes it clearer--the pink is all surfaces that will contain the concrete).
(If you are using a typical template your front faces are probably white and the back faces are blue. My front faces here are the pale yellowish color.)
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