Extending a schema by adding choices

We are trying to define 2 schemas, one which allows 2 options in an
xsd:choice, and one which extends this and allows 3 options.
If one schema defines 2 choices of elements, can this list of choices be
extended in a deriving schema?
eg:

<xsd:complexType name="Contact">
  <xsd:complexContent>
    <xsd:choice>
      <xsd:element name="Telephone" type="xsd:string" />
      <xsd:element name="Fax" type="xsd:string" />
    </xsd:choice>
  </xsd:complexContent>
</xsd:complexType>

Allows for example:
<MyContact>
  <Telephone>(123) 456 7890</Telephone>
</MyContact>
<MyContact>
  <Fax>(987) 654 3210</Fax>
</MyContact>

I have used the basic xsd:string type for example only.

Is it possible to also allow the following in a deriving schema:

<MyContact>
  <EMail>Bill@Place.com</EMail>
</MyContact>

My guess is that this can't be done, but I wonder if the <xsd:extension>
element can be for this type of definition...

Thanks,
Daniel Flippance
Senior Software Engineer
Meridian Project Systems