<li class="fields">
                    <div class="field">
                        <label for="billing:ssn" class="required"><em>*</em><?php echo $this->__('SSN') ?></label>
                        <div class="input-box">
                            <input type="text" name="custom[ssn]" value="<?php echo $this->htmlEscape($this->getQuote()->getSsn()) ?>" title="<?php echo $this->__('SSN') ?>" class="input-text required-entry" id="billing:ssn" />
                        </div>
                    </div>
 
                </li>

http://www.excellencemagentoblog.com/magento-add-custom-fields-checkout-page

In this blog, we will see how to add custom fields to magento onepage checkout steps.
This method can be used to add custom field to any step in the checkout page, although in the current blog i have demonstrated how to add it to the billing step. This method will work on all magento versions.

Magento Checkout Page - Add Custom Fields

magento

PHP