SDS.AM Extensions

I like the new System.DirectoryServices.AccountManagement namespace. It
seems like it will make a lot of things easier to manage.

I am having trouble understanding how to extend and use the UserPrincipal to
get some things done. These include searching and updating the
proxyAddresses attribute for users. I tried to do this by Extending the
UserPrincipal as follows:
[DirectoryObjectClass("User")]
[DirectoryRdnPrefix("CN")]
public class Account : UserPrincipal {

// Constructor
public Account(PrincipalContext ou, string samAccountName, string password)
:
base(ou, samAccountName, password, true) { }

[DirectoryProperty("proxyAddresses")]
public string[] ProxyAddresses {
get {
int len = ExtensionGet("proxyAddresses").Length;
string[] addresses = new string[len];
object[] addressesRaw = ExtensionGet("proxyAddresses");
for (int i = 0; i < len; i++) {
addresses[i] = (string)addressesRaw[i];
}

return addresses;
}

set { ExtensionSet("proxyAddresses", value); }
}

I can use this to create a new account (User) in the AD but cannot use it to
retrieve the proxyAddresses. The various find methods only return
Principals or UserPrincipals and I cannot cast them to Account. I am using
http://msdn.microsoft.com/en-us/library/bb552835.aspx as a model to work
from.

How can I access/update/search the proxyAddresses field for users?

Thanks, bob