The short of it, you are fine use them.

The longer version. Because the extensions are most like AL wire, or tinned. When you attach them to copper and they will oxides faster. Now that being said, the oxidation will take awhile and the way shit is made these days, the wire will most likely outlast the door bells. But if you want to extend the life of the wire, you can buy Noalox (or some type of anti-oxidant inhibitor) to add to the wires when you make the connection to prevent this.