This is a common request, and support for booru sites is planned!
HDoujin Downloader is currently specialized to work well with manga/doujinshi websites, but boorus require some special considerations due to their large number of images (e.g., image limits, per-image tag support, etc.).
You can request support for a booru site, but it will not be added until booru support is fully-implemented. However, I’ll add the site to my to-do list so that when booru support is added, I can be sure to add support for the site!