Unfortunately, this does not work in Cubase. You need to create a separate stereo track for that.
I prefer having stereo tracks separated on two mono tracks hard panned left/right, because you’re more flexible to edit them. If you wanna try that out, there is an option to split stereo tracks to L/R mono tracks when importing an audio file.
But there is a feature request to convert mono tracks to stereo and vice versa, you can support that one with a vote (on the left of the thread title) and the ones with the most votes are more likely to get implemented by Steinberg.