Standard DLC NSPs from the eShop will not work unless you have a DLC Unlocker or a titlekey in your prodsystem.ini (Atmosphere) or use a CFW that bypasses ticket validation. On Atmosphere with SigPatches, just install the DLC NSP like a normal file.
Third-party tools and NSP files from untrusted sources can contain malware or other security threats, compromising the integrity and security of the Switch console. my hero ones justice 2 switch nsp update dl install
If you want to how NSP updates work (for homebrew or modding research), search these terms (not links): Standard DLC NSPs from the eShop will not
The Nintendo Switch uses a file format known as NSP for its digital games. NSP files are essentially containers that hold all the data needed for a game to run on the Switch, similar to how cartridges and digital downloads work on other Nintendo consoles. These files are typically downloaded from the Nintendo eShop or other digital distribution platforms and then installed directly onto the Switch. If you want to how NSP updates work