Is it possible to use custom maps on Sigma ROX 12? I wonder if it's possible to somehow upload custom maps to Sigma ROX 12 bike computer? The default map that is available on it for my country is not very good, so I want to customize it.

p.s.
When I say "maps" I mean maps and not routes.
Looking through the specs of the ROX12 it should be possible to store custom map data on a compatible SD card. 






The device itself seems to use OSM format, this is Open Street Format that can be downloaded from either 
Open Street Map

Or 

More Open Street Map Info

According to the manual ROX12 User Manual you should be able to select custom maps from within the device. You will however need a PC of some sort to download the maps and place them on the SD Card, also note the maximum SD Card is 128GB so you want your custom maps being to big.
The advantage of using the first Open Street Map website i linked to is you can pretty much pick any size map, if it gives you an error of maximum export size exceeded you can easily make the map smaller just by dragging your mouse over the page.

Hopefully this helps the OP achieve what they need.