It looks like you can do it that way according to the docs [0] :
> abroot pkg add PACKAGE_NAME
Not sure what it does exactly under the hood. I'm not sure it persists after an upgrade.
In the release announcement blog post they also mention a way to build your own (base OS?) image with something called VIB [1]
Regarding space efficiency, the distribution relies on a "LVM Thin provisioning" feature. [2]
I don't know enough about it (nor about ostree) to compare the two.
This is all very refreshing! Many new techs and concepts to look into :)