There are a number of areas in WSN which are used to send an email based on information typed into a form -- the email article page, email member page, email submitter, contact form, et cetera. Suppose you want more than the generic "type your message" box. Just type your new form field in the template, and edit the corresponding email to use {FORMFIELDNAME} (that's {FORM + FIELDNAME + }) to display the value the user supplies where you want it displayed in the email.

As an example, here's how you'd add a dropdown of "department" options for the contact form:
1) Edit the contact form template to insert your field:
<select name="department">
<option value="Human Resources">Human Resources</option>
<option value="Information Technology">Information Technology</option>
<option value="Other">Other</option>
</select>
2) Go to Admin -> E-mails -> Edit Outgoing E-mails and edit email_contactformbody to add {FORMDEPARTMENT} where you want it displayed.