I have a software (I can access to the source code) which is compiled
in dll with Compaq Visual Fortran. I want to create an external dll
which will be called by my software.
The aim of this manipulation is to allow User defined function: allow
user that have not the permission to access the source code of the
software, to create their own function.
I understand that I have to add
cDEC$ ATTRIBUTES DLLIMPORT:: function_test
in the source code of my software
and in my external dll i have to add:
cDEC$ ATTRIBUTES DLLEXPORT:: function_test

But I don't understand how to build my external dll. I have the linker
error 2001.

The procedure is described in the online Programmer's Guide - under
"Creating Fortran DLLs".  From the command line you have to remember the
/dll command line option when creating the DLL,

e.g. DF /dll mydll.f90

and to include the .lib file that command creates when linking the
calling program.

e.g DF mainapp.f90 mydll.lib

The .lib file is a glue layer that tells the main program how to get at
the DLL.

There are special cases, and the procedure is different if you are using
Visual Studio, but it's all described in the Programmer's Guide.