Adding global signature for outgoing mails
It's possible to configure WebMail Lite installation so that the same message is appended to all the outgoing mails for all the users of the installation.
To do that, you need to install the following module: Global signature plugin
NB: This doesn't affect per-user signature which can be set by users themselves in Settings > Email Account screen under Identity tab.
Instructions for installing a module can be found at Adding modules documentation page.
However, since this particular module doesn't have a front-end part and is purely server-side, there's a simpler way which doesn't require using Composer, Npm and Gulp:
Now you can supply the signature in data/settings/modules/MailGlobalSignaturePlugin.config.json
config file, "Signature" setting.