It’s usually done by converting a DSK image to a CPR using Nocart
This process is pretty straight forward but many games also need some (manual) patches so you don’t need a keyboard and can play it with the joypad only.
Then you have another versión that convert the dsk to cpr and you can add a external .bin
for example you found a game heroquest that load the menu,but you can’t load the levels after converting It,then you can made a loader for the levels in one Page, and extract the levels manually apart. Then you can take all parts and glue It with copy command of Windows into a .bin,upload the .bin to the cartridge with nocart, and you can access that .Bin from Page x to x in the cartridge.
Another versión of no cart extend the size dedicated the dsk part, and reduce the size of the .Bin part.
Should be another tool there where you can take roms and you can select in what cartridge slot you can put It. But i don’t remember now where is it.