Merge Field Variables

Hi

Are we restricted to using the only variables. I have a first name field, in my form that I would like to use and wondering what the expression I should use is.

{username} – The user’s username on the site
{user_email} – The user’s email address
{sitename} – Your site name
{site_url} – Your site URL
{admin_approve_url} – The URL to approve/deny users
{login_url} – The URL to login to the site
{reset_password_url} – The URL for a user to set/reset their password
{password} – Generates the password for the user to add to the email