On my machine darkan's extension.yml file does not work. Repeating the 'extensions:' line means the original one is ignored.

This should work:
---
Name: extensions
---
Member:
extensions:
- MyMemberExtension
- MemberProfileExtension

cheers
Bruce

Hey! very later ansxer... Sorry!

---
Name: extensions
---
Member:
  extensions:
    ['MemberProfileExtension']
  extensions:
    ['MyMemberProfileExtend']

Hello all, i have a new question for this modules and add fields.
I try to explain with my very bad english... sorry.
So, i use SS_3.1.8 with the MemberProfileExtend off ajshort and all is fully fonctional.

Now, i would like to add un "pre-form", before to send the normal member profil form to register.
-> add a first form with new fields,
-> call another database and return value (dont need to register in the member class),
-> save value in session::
->go to normal form of MemberProfil to register, and insert some default session value.
-> send mail to validation.

Here is what i'm do :

I know in it's don't good to insert in core (but it's only for test):

/memberprofiles/code/MemberProfilePage.php

class MemberProfilePage_Controller extends Page_Controller {
	private static $allowed_actions = array (
		//ajout formulaire_2
		'VerifFamIDForm',
		//
		'index',
		'RegisterForm',
		'afterregistration',
		'ProfileForm',
		'add',
		'AddForm',
		'confirm',
		'show',
	);
	/**
	 * My first form - Verif Fam_ID
	 */	
	public function VerifFamIDForm() {
            $fields = new FieldList(
                new TextField('fac_id', 'Num. Facture'),
				new TextField('fac_sum', 'Montant')
            );
            // Create actions
            $actions = new FieldList(
                new FormAction('doVerifFamID', 'Vérifier')
            );
			$required = new RequiredFields('fac_id', 'fac_sum');
			//
            return new Form($this, 'VerifFamIDForm', $fields, $actions, $required);
    }
	//
	public function doVerifFamID($data1, Form $form1) {
			//recup mdb
			global $databaseConfig, $databaseAutre;
			DB::connect($databaseAutre);
			//
			$sqlQuery = new SQLQuery();
			... bla bla bla
			$result1 = $sqlQuery->execute();
			//
			foreach($result1 as $row1) { 
				Session::set('fam_id', $row1['bp_family']);
				Session::set('fam_mail', $row1['per_email']);
				Session::set('fam_name', $row1['per_name']);
			}
			//
			DB::connect($databaseConfig);
			return $this->indexRegister();
	}
	//...other code...
}

memberprofiles/code/MyMemberProfileExtend.php

<?php
class MyMemberProfileExtend extends DataExtension {
	private static $db = array(
		'Identifiant' => 'Varchar(30)'
	);
	//
	public function updateMemberFormFields(FieldList $fields) {
		$fields->push(new TextField('Identifiant', 'Identifiant', Session::get('fam_id')));
		//$fields->ReplaceField('Identifiant', new TextField('Identifiant', 'Identifiant F', Session::get('fam_id')));
		$fields->ReplaceField('Email', new EmailField('Email', 'E-mail F', Session::get('fam_mail')));
		$fields->ReplaceField('Surname', new TextField('Surname', 'Nom F', Session::get('fam_name')));
		
	}
	//
	public function getCMSFields() {
		$fields = parent::getCMSFields();
	    $this->extend('updateCMSFields', $fields, $actions); 
	    return $fields; 
	}
}

/memberprofiles/templates/Layout/MemberProfilePage_register.ss

<% if session_fam_id %>
	<p>It's ok, you can register now</p>
	$Form
<% else %>
	<p>Not ok !</p>
	VerifFamIDForm
<% end_if %>

All it's ok. But when i receive the registration mail to confirm, there is no "...?key=" key value (empty).
So in the database member, the new member have an empty TempIDHash (NULL) and no ProfilePageID (0).
All other fields are good. The new field "fam_id" too. It's ok.

I'm lost in code... Please, if just you have an idea ...?!
Thanks to read.