If you mean booking an award open sleeper ticket, maybe. I imagine they might be bookable at what appears to be the standard point redemption value of $0.0267/point. I can well imagine the reservation system is incapable of handling an open sleeper award ticket, too, though. Bottom line is you would have to call and try to do it through an AGR agent anyway, so talk to them.

Be aware that an open sleeper ticket and a coach ticket are not the same thing. For one thing, open sleeper ticket cost is the sleeper rail fare. That fare is usually higher than the available coach fare in recent years. Some years ago, it was the same as the lowest coach "value" bucket, so equivalent to the available coach fare, occasionally lower if coach fares themselves had moved into higher buckets. There days it is around the middle coach bucket and coach itself is usually available in lower buckets.

Also, I think the sleeper reservation holder may have to be the one that requests an open sleeper. At the very least, you need that reservation number (PNR) to be able to book an open sleeper along with reservation holder's name.