simply make a new template of the initscript, I mean this
/etc/e-smith/template/etc/rc.d/init.d/arpwatch/10base
then of course expand it with nethserver-arpwatch-{update,save}
you will need probably to set owner ship and permisions : Templates — NethServer 7 documentation
create a file /etc/e-smith/template.metadata/etc/rc.d/init.d/arpwatch with the relevant content
for example
UID="root"
GID="root"
PERMS=0755