Adding new user info to current old user accounts

Hey all so I have a database that has been up and running for a few months now. I had a request to add the users First and Last name as well as their organization. This was easy enough to do:

"INSERT INTO " +     "aspnet_UsersAddition " +         "(UserId, firstName, LastName, OrganizationID) " + "VALUES " +     "(" +         "'" + genGUID + "'," +         "'" + Params.firstName + "'," +         "'" + Params.lastName + "'," +         "" + Params.Params["Org"] +     ")"; 

However, and this is the problem, is that OLD users who do not have that First/Last and organization associated with their account need to have those added to them.

I have tried the following:

INSERT INTO      aspnet_UsersAddition            (UserId, firstName, LastName, OrganizationID)  VALUES      ('','','',1)  WHERE      UserId = (SELECT UserId FROM aspnet_Membership WHERE Email = 'admin@admin.com'); 

But of course the above would not work since the UserId in the aspnet_UsersAddition was non-excitant when comparing it to the aspnet_Membership UserId. The only info I have access to on the page where I am calling this function is their email address and user name.

So I guess my question is – How do I add a new record using the users GUID [that needs to be looked up via email address or user name] so that I can add that info [UserId] to the new table [first/last and organization]?

Do I have to make separate query’s

Select old users GUID using their email address.

Insert new record into new table with blanks other than the GUID.

in order to do this? Or is there a way to do this in one query?