To create a contact form that will allow your users to sign up to a mailing list when they want, but to be able to use the subscription form as a contact form whether they want to sign up or not simply follow these simple steps:

Create two mailing lists:

1. General Inquiries
2. Newsletter

Create a sign up form that allows you to sign up to both mailing lists and emulate a contact form. Change the General Inquiries to a hidden form field in the HTML.

Now, when a user fills in the contact form, they'll be given the option to sign up to the newsletter or not. The General Inquiries will always collect their information, so you can do whatever you wanted with it (try maybe an autoresponder to follow up with them after a month or so? eg. "hi %%Firstname%%, Just wondering if there was anything else I could help you with?") and the newsletter option will have people who want to be on your newsletter.