How do I deploy a stack script to an existing linode configuration?

How do I deploy a stack script to an existing linode configuration that only wipes the distro's root partition, instead of creating the entire linode anew?

I've looked all over the cloud interface and through the docs, and unless I'm missing something obvious, there is no longer a way to deploy a stack script to an existing configuration. (Since we've moved to the new "cloud" admin U/I, that is - I very specifically remember doing this in the old U/I). Every way I can see to deploy a stack script results in the existing configuration and partitions being wiped clean and the default distro configuration - in this case for archlinux - is used, with brand new single root and swap partitions created.