Can be EmbOS added to STM32CubeMX as Middleware SW module?

​Yes but ST does not directly provide the related support of EmbOS on STM32WB devices.

This support is done through a partner, here it is Seeger, which provides solution to port EmbOS but you will have to go through a licensing process to get the source files. No idea about any potential cost.

You can also get more info on the ST web page below.

https://www.st.com/content/st_com/en/products/embedded-software/mcu-mpu-embedded-software/stm32-embedded-software/stm32cube-expansion-packages/i-cube-embos.html