You can just [control] drag the mono audio to a new empty stereotrack. (Use [control] for not moving the file in time). Move your plugins to the new track by dragging them in the mixer, while displaying inserts, or create a mixerpreset of the old settings if you already had plugins and sends on it. When using Waves plugins for example create a preset in the plugin preset manager (not the cubase preset manager). Then recal the stereoversion of the plugin, and recal the preset.
To start mixing mono files from another studio on stereotracks;
You could create (or copy) a template with only stereotracks, fx returns, groups, and your master plugins, so already loaded with plugins and all.. Then import (drag) the mono audio to NEW mono tracks (on the bottom of the arangement), then put all the files were you want them on the stereotracks, then delete the unused monotracks and start mixing..
A mono file will just play normaly on a stereotrack, it's the plugins that are usualy different so you will always have to change those into stereoversions..
You can also move your mono plugins to the stereo-channel (loaded with a mono track), and insert a "mono to stereo" reverb or other effect after them, and from then on stereo effects in the chain.
You can record a mono wav file on a stereotrack just by selecting a mono ASIO input. The waveform and the file will be mono. So no need for mono tracks actualy...(I rarely use them).
Files from other studio's often have "dual mono" stereotracks (stereo sounds on 2 mono tracks panned left-right) Those you will have to bounce to a stereo-interleaved file to be able to import them on a stereotrack. I have a "bouce" output set to 0dB routed to no ASIO output for that in my standard template.
Studio: Cubase 6.5 Windows XP Pro SP3 32 bit / Cubase 7.5 Windows 7 64 bit, Intel Q6600 @ 2.40Ghz / Asus P5K Motherboard / 3gbswitch for XP / Speedstep off / 4 gigs Corsair DDR2 RAM @400mhz / NVIDIA GeForce 850 GT / 2x RME Digiface PCI 26in/out, 1x RME multiface PCI
Laptop: Cubase 6.5/ Windows XP Pro SP3 32 bit/ Dell LAtitude E5500 / 4gb/ RME Digiface via Express Card V2
Ipad2/16gb, Lemur running customized 16channel Mackie control and multiple generic remotes template, presonus faderport, BCR2000, 2x BCF 2000, 14 bits accelerated controlers on Lemur and BC's