Allow Duplicate email addresses

I am using the current releases of both WP and WP-M. I have a registration page for new members to apply using the WP-Members shortcode [wpmem_form register]. This displays fine and works well. I also have an old plug-in installed, called ‘Allow Multiple Accounts’ which allows multiple user accounts to be created, registered, and updated having the same email address. This is necessary because we have family members where young children use their parents’ email address. This plug-in is old, but it does work in a case where a new member is registered by an admin using the WP Admin UI (i.e. not involving WP-Members). However, if a new member tries to join using the WP-Members form, and uses a pre-existing email address, they are presented with a warning that ‘This email address is already in use’ and can go no further. I’m wondering if there is a hack or function I can add to my theme’s functions.php to by-pass this problem. I understand that this restriction is supposed to be a security precaution and I don’t want to tamper with either WP or WP-M core code if it can be helped.

Any help/suggestions appreciated.