I've never heard of a stroller add on and I've had APs for the past 2 years. Of course my girls are older so that may be why it wasn't offered to me.....
Apparently they used to let you add on a stroller pass which was good for one stroller a day. It was the same as adding parking onto a pass. I haven't read anything more recent than a few years ago, so maybe they dont offer it anymore?