form 属性を使用する事で、form要素外の input 要素を使うことが出来ます。
<form action="action_page.php" id="form1"> First name: <input type="text" name="fname"><br> <input type="submit" value="Submit"> </form> <p>The "Last name" field below is outside the form element, but still part of the form.</p> Last name: <input type="text" name="lname" form="form1">
http://www.w3schools.com/html/tryit.asp?filename=tryhtml5_input_form(b:title)
http://caniuse.com/#feat=form-attribute(b:browser support)