You can add library files directly to the Simply Fortran project by selecting "Add Files..." from the Project menu.  However, I should warn you that .lib files are not native to Simply Fortran's GNU compiler, and they may not work directly.

Are the library files provided simply for linking to DLLs?  If so, you can also add DLL files directly to the Simply Fortran project, and the compiler should be able to link appropriately without using the .lib files.

I used to compile a project with open watcom. Every module needed library files to work. On open watcom, I just had to add my *.for file, and all my *.lib files, and it worked.

How can I do the same with Simply Fortran? What is the way to include *.lib files?
In this files, which are lib archive, there are few .dll files with an unknown contents (that's not my project).