The XML conversion system provides methods and architecture for defining an interface between a spreadsheet application and a COM server independent of the areas that the interface is defined. In support of defining the interface, an add-in function defines the interface using extensible markup language (“XML”). An add-in XML file is scanned for instructions. An instruction is analyzed to determine the interface it will expose. An interface XML file having a function specification can be retrieved from a COM server based on the interface exposed in the instruction. Function qualifiers are taken from the instruction and applied to the function specification. The function specification is converted to an intermediate function and implementation specifiers are applied to the intermediate function. The intermediate function is then converted to an add-in function and applied in the spreadsheet application.