Yeah, I guess. This single button above had something like this. It could emit programmed combinations of keys instead of single key-presses. So for example if you wanted to have "µ" you would program left alt + m, for Polish "ó" you would program left alt + o and so on. I am talking about a prototype of course, because production is another story.

can You create this? https://klawiatura.wordpress.com/

In terms of programming this is definitely doable. Every MCU vendor provides a hid example these days so I'd start there. But recently i became a big fan of zephyr rtos which also provides a hid sample so if you're into operating systems this is the good place to start as well. Sorry for replying after 3 years, but i missed your comment.