In BigSIS we don't have email signatures per se, but we do have templates. What you can do is create a template that has your signature in it, and then always use that template when sending emails.
How to Add Signatures to Emails in BigSIS
To add a new template, click the blue "+" button. To edit existing templates select the template then click the green edit button.
If you have existing templates you can add the signature to those as well. While sending an email, choose "From Template" in the "Type of Email" field.