You will have to make changes in each controller, where you'd like to insert email from session.
For Login page open /catalog/controller/account/login.php
find
if (isset($this->request->post['email'])) {
$data['email'] = $this->request->post['email'];
} else {
$data['email'] = '';
}
Replace with
if (isset($this->request->post['email'])) {
$data['email'] = $this->request->post['email'];
} elseif (isset($_SESSION['useremail']) {
$data['email'] = $_SESSION['useremail'];
} else {
$data['email'] = '';
}
Now you will get your email from $_SESSION['useremail']
on login page.
If you wish to add those email anywhere else on your OpenCart website - you will have to rewrite $data['email']
in related controllers the same way as above.