Technically, HxD does not have a native, user-facing plugin API (like a .dll folder you can just drop files into) in its current stable releases. However, the developer has designed the tool to be highly extensible through and Data Inspector configurations.
Some popular HXD plugins include:
The plugin framework is designed to let developers create their own Data Inspector types. Instead of just seeing raw hex bytes (e.g., 48 65 6C 6C 6F