Compiling all files from a predefined directory with Arm Keil Studio Pack (MDK v6)

Hi,
I'm using the Arm Keil Studio Pack (MDK v6) with Visual Studio Code (v1.109).
And want to start a new project while still using an older library.

My project structure is as follows:
ProjectsDirectory/
    NewMDKv6Solution/
        NewProject/
            main.c
            NewProject.cproject.yml
    OldStillUsedCodebase/
        include/
            library.h
        src/
            library.c

In main.c:
#include "library.h"

int main(void) {
    library_init();

    for (;;);
}

In library.h:
#ifndef LIBRARY_H
#define LIBRARY_H

void library_init(void);

#endif

In library.c:
#include "library.h"

void library_init(void) {
    // ...
}

In NewProject.cproject.yml:
project:
  groups:
    - group: Source Files
      files:
        - file: ./main.c

  add-path:
    - ../../OldStillUsedCodebase/include
    - ../../OldStillUsedCodebase/src

When I now try to build my project, I get a linker error stating that `library_init` cannot be found.
After looking at the outputted files, I noticed that `library.c` did not get compiled.
So I edited `NewProject.cproject.yml` to use this file as a source file instead.

In NewProject.cproject.yml:
project:
  groups:
    - group: Source Files
      files:
        - file: ./main.c
    # This would be tedious with 100+ files
    - group: Library Files
      files:
        - file: ../../OldStillUsedCodebase/src/library.c

  add-path:
    - ../../OldStillUsedCodebase/include

Is there a way to declare an entire directory, where all the files inside should be compiled?
Or maybe even that I can define a filepath pattern like ../../OldStillUsedCodebase/src/*.c somehow?

Thank you in advance.