Hi,
I found a working solution for registration and update with profil.
1- created a Chunk [[$mydropdown]] with my options, where [[+cdz]] is my extented profile field :
<select class="myclass" name="cdz">
<option selected="[[+cdz]]">[[+cdz]]</option>
<option value="myvalue1">myvalue1</option>
<option value="myvalue2">myvalue2</option>
</select>
2 - insert that chunk [[$mydropdown]] in both register and updateProfile pages.
3 - check that when creating a new account form the register page the dropdown list is empty by default, and select an option
4 - check that in the updateProfil page, when logged in, the dropdown value shows the value entered at step #3.
That's it ! Hope that basic solution will help !
Notice : at step #4 the first value of the dropdwon list will also appears in the list at his normal place, as this tip isn't dynamic.