Type libraries are quasi-replacements for XLL add-ins. The type library can contain constant and function declarations and other information about the DLL. Once you have created a type library, you can use the References command on the Tools menu in Microsoft Excel to load the type library. Once loaded, the constant and function declarations are available in Visual Basic without using Declare statements. For more information on creating type libraries see the OLE Programmer's Guide Volume 2 or MSDN.
Visual Basic in Microsoft Excel does not support user-defined data-structure declarations in type libraries (user-defined data structures are those declared with the Visual Basic Type statement or the C/C++ struct statement). To use the structures in your Visual Basic code, you will have to add their declarations to the module.