How to get instance of extended object at runtime

[softwareguy74]

I'm not even sure the title correctly explains the situation that I'm running into, but I'll try my best to describe it.

I've got the following classes defined:

public class AccountBase
public class ExtendedAccount1 : AccountBase
public class ExtendedAccount2 : AccountBase

I have a method such as:

public void AddAccount(AccountBase account)
{
     var o = <convert account into actual extended account type>  <-- how to do this?
     thirdPartyLibrary.DoSomething(o);
}

Somehow in AddAccount, I need to be able to create an instance of the 'account' object as the extended type that was passed in, without having to resort to an if or switch statement to determine what type 'account' actually is.

The reason I need to do this is because third party library that I'm using is only "seeing" AccountBase when I pass in ExtendedAccount1 or ExtendedAccount2. I need it to actually "see" the full extended class.

DoSomething is defined as:

public T DoSomething<T>(
T value
)

I know I can do something like:

public void AddAccount(AccountBase account)
{
     if(account Is ExtendedAccount1)
     {
           thirdPartyLibrary.DoSomething((ExtendedAccount1)account);
     } else if(account Is ExtendedAccount2)
     {
           thirdPartyLibrary.DoSomething((ExtendedAccount2)account);
     }
}

But I REALLY don't want to do this because then I would have to maintain this block of code every time I added a new extended account type. I'm really hoping to make this as generic as possible.

Is this possible? Example code would be greatly appreciated.