A file named lgrlist.txt can be placed in the lgr directory to change the lgr used in internals. The following information comes from elma_readme_dk.txt, written by Balazs Rozsa:
The LGRLIST.TXT file
If you create an lgr file that contains pictures with the same names that are the names of the pictures in the original default.lgr file, you can replace the original default.lgr file with your own file. This way when you play on the internal levels, the biker, the trees, the bushes and every picture will be replaced by your objects.
If you do not want to replace the original default lgr file for all levels, you can do it, just create an lgrlist.txt file: Let's say you make your own file, called myfile.lgr, and you copy it to the lgr subdirectory. You create a file also in the lgr subdirectory, called lgrlist.txt, which has the following lines: 5 myfile.lgr 6 myfile.lgr 8 myfile.lgr
In this case if you play on internal level 5, 6 and 8, your lgr file will be loaded.