Cascade, if I understand right, not in the way you'd like.

You can book a roundtrip with stopover O(rigin) - D(estination) - S(topover) - O(rigin).
For one ways, you can't have a stopover. So you'd have to book D-S as a separate paid or points ticket. If D and S are both in Europe, you *may* be able to get a nonrefundable economy ticket quite inexpensively.

What you could try is book a dummy return right when O-D or O-D-S opens up, so you have O-D-S-X-Y where X is an optional crazy connection Y is any open jaw destination (not necessarily O) that is available and works within the norms of the return Aeroplan ticket. Then phone back later and switch S-X-Y to S-O. It would cost you a change fee.

You're probably aware of this, but stops of less than <24 hrs (there may be exceptions) count as connections not stopovers. So I assume you want S to be a longer stopover.