YOu can't.
But ... You could store the DataAdapters in a Dictionary and access them that way:
Dim adapters As Dictionary(Of String, SqlDataAdapter) = New Dictionary(Of String, SqlDataAdapter)()
adapters.Add("Customers", New SqlDataAdapter())
adapters.Add("Invoices", New SqlDataAdapter())
...
Dim tableName As String = "Invoices"
adapters(tableName).SelectCommand = New SqlCommand("SELECT * FROM " & tableName, con)