Using SQLDataReader to insert multiple rows in table

I'm probably going about this the wrong way, but here goes:

I am starting a club database in SQL server that includes ASP login functionality. I have been given all the members' details in a spreadsheet and am trying to import every Club member into the database with their own login.

I have imported the spreadsheet to a table called Historical$ and have used Peter Kellner's Membership Editor to set up the ASP Membership tables in my database. Works great!

Therefore I have all my club "members" as ASP Membership with their own login details but now want to put all their info (address, gender, etc) in a table that uses the UniqueIdentifier "UserId" from the ASP Membership tables as a foreign key.

In short, I want to import selected fields from my source table (Historical$) + the corresponding UserId from aspnet_Membership table into an empty table called "Club_Member". I would prefer to use the structure that I already have in Club_member if possible.

Source table Historical$:

Mem_no  
Firstname  
Lastname  
email1  
...  

Source table aspnet_Membership:

...  
UserId  
...  
Email  
...

Target table Club_Member:

UserId  
Firstname  
Lastname  
Email  
...

I thought that the best way would be to use a SQLDataReader to select the fields with a table join, read each row in the results and insert into the new table, but having trouble working out the logic and syntax to insert into the new table.
C# Code:

protected void ButtonTransfer_Click(object sender, EventArgs e)
    {

        SqlConnection con = new SqlConnection(connectionString);
        string strSQL1 = "Select Historical$.memnum, Historical$.first, Historical$.last, aspnet_Membership.UserId FROM aspnet_Membership INNER JOIN Historical$ ON aspnet_Membership.Email = Historical$.email1";
       con.Open();
         SqlCommand myCommand = new SqlCommand(strSQL1, con);

        SqlDataReader TransReader;
        TransReader= myCommand.ExecuteReader();
        while (TransReader.Read())
        {

            SqlDataSource1.Insert();
        }
    }  

ASP Code:

        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
        ConnectionString="<%$ ConnectionStrings:wtcConnectionString %>" 
        InsertCommand="INSERT INTO member(UserId, mem_number, mem_fname, mem_lname) VALUES (@UserId, @memnum, @first, @last)">
        <InsertParameters>
            <asp:Parameter Name="UserId" />
            <asp:Parameter Name="memnum" />
            <asp:Parameter Name="first" />
            <asp:Parameter Name="last" />
        </InsertParameters>
    </asp:SqlDataSource>        

I am getting the error: Cannot insert the value NULL into column 'UserId', table Club_Member; column does not allow nulls. INSERT fails.

First question is, Am I on the right track, or is there a better way?
Secondly, What might be causing the error? I don't mind if someone comes up with an easier way because this is supposed to be a "one-off" but I don't get how to go about it! (BTW ASP.NET/C# knowledge limited, SQL knowledge very limited!)

Thanks, and sorry about the essay.