There might be a simpler way, but when I was faced with the same situation, I booked an award stay as far out in the future as I could book. Then, I credited an actual stay to the account before canceling that award some time down the road. The points from the cancelled award went back into my account without expiring. Granted I did this a while ago, so I am not sure if the points would be "assigned" with the original expiration when the award gets canceled now.

LAX