100% agree. Such a built-in feature for file manipulation would be great. In theory you could do this via CLI, but if you’re a bash newbie like myself it can be kinda daunting. It’s also one of main reasons why I got into writing my own scripts.
In the meantime, you might want to keep an eye out on my upcoming extension. One of its features is a file merger which lets you import all the open files in Aseprite (from imported sprite sheets to simple frame animations or complex stuff with tons of layers/layer modes, etc). The tool is mostly done but I’m still debugging it with my own project files to make sure nothing breaks. If you haven’t imported the sprite sheets (that is, split into frames), you’ll be able to use it in combination with this bulk sprite sheet imported. You can check the documentation for the file merger here. Once it’s ready I’ll probably post some links in the #scripts board.