How can I add custom meta on signup page and pass along to be used after blog activation?

I know it’s a very old question, but I had the same issue and this was never resolved so I thought I’d post a solution. I found the answer in a different topic here on WPSE.

You only have to use the filter add_signup_meta.

function sb_add_signup_meta($meta) {
    // hardcoded value
    $meta['mykey'] = 'My Value';

    // $_POST value from register form
    if ( isset( $_POST[ 'value' ] ) && ! empty( $_POST[ 'value' ] ) ) {
        $meta[ 'meta_key' ] = $_POST[ 'value' ];
    }

    return $meta;
}
add_filter('add_signup_meta', 'sb_add_signup_meta');