Apparently they removed the ability to change the number of audio channels in a track after it’s created because it caused issues while they were expanding the routing capabilities of Cubase and Nuendo. It’s something more complex than it seems.
The standard procedure in Cubase when working with mono audio is to either drag it to a stereo track or use Group Tracks or FX Channel sends when stereo effects are required. Most stereo effects are stuff that works better as sends anyway, like reverb, delay and chorus.
Another option is to just batch convert these mono files to stereo before working on these old files exported from Logic. This can be done using the Pool Window in Cubase. Import your audio files to a dummy project, open the Pool Window (Ctrl+P), then select everything and convert the files to Stereo Interleaved. This will create separate stereo versions of all mono files. Anything that’s already stereo will be left unchanged.