There is a "-s" argument to the "rootfs-image" image you are using to inject state scripts,
$ mender-artifact write rootfs-image --help
NAME:
mender-artifact write rootfs-image - Writes Mender artifact containing rootfs image
USAGE:
mender-artifact write rootfs-image [command options] <image path>
OPTIONS:
--update FILE, -u FILE Update FILE.
--device-type value, -t value Type of device(s) supported by the update. You can specify multiple compatible devices providing this parameter multiple times.
--artifact-name value, -n value Name of the artifact
--output-path value, -o value Full path to output artifact file.
--version value, -v value Version of the artifact. (default: 2)
--key value, -k value Full path to the private key that will be used to sign the artifact.
--script value, -s value Full path to the state script(s). You can specify multiple scripts providing this parameter multiple times.