EDIT:
Right, I just found out myself: Adding
/^(entry[\d-]+)\.txt.gz$/ && system("gunzip -fv $dir/$_");
after line 623 in import_dir unzips the postings. I had to rerun the script afterwards to do the import, I’m sure there is a way to do it in one step, but I don’t know much Perl and that did the trick for me. Thanks for the cool script!