What design/pattern to use for a Client application using multiple providers?

This is a design related question.

Lets say we have a public API called ClientAPI with a few web methods like CreateAccount, GetAccount. Depending on the customer, we use a number of different providers to fulfil these requests.

So say we have ProviderA and ProviderB and ProviderC.

ProviderA has a method signature/implementation of CreateAccount that needs (Firstname, Lastname) only and creates an account with ProviderA.

ProviderB has a method signature/implementation of CreateAccount that needs (Firstname, Lastname, Email, DOB) and creates an account with ProviderB.

ProviderC has a method signature/implementation of CreateAccount that needs (Nickname, CompanyKey, Email) and creates an account with ProviderC.

The Client doesn’t need to know or care about which provider they are. When the Client API method CreateAccount is called, the client api will work out what provider(s) it needs to call and invokes that Providers Method.

So there are two questions I have here.

1) What is the best design/pattern to implement for this model? Also bearing in mind that the number of providers will grow – we will be adding more providers.

2) Regarding passing parameters – currently the ClientAPI CreateAccount method signature is a big line of variables, and if a new provider needs a new value, the method signature has another variable added to it, which obviously breaks the old implementations etc. Is it a good practice to pass an array/list/dictionary of parameters in the method signature and into the providers below, or is there a better way?