You can call the same function from within the DLL and from an external app that calls the DLL. You don't need to create a new function to do this.
I create a separate .h file for functions I want to export. I
copy prototypes from the internal .h or .c files to the export .h file and change the type to extern int __cdecl. I add the export .h file to my DLL project, but I don't #include the export .h file in any .c or .h file in my DLL project. I #include it in my app that calls the DLL functions. See additional details in my answer
here.