Why do you want them in stereo? Generally you would leave them as mono tracks and just mix everything to a single stereo track at the completion of your project.
However, you can convert single tracks to stereo by selecting the specific track and then use the Bounce to Track command. You will get a selection menu when you execute the command where you can choose "mix to a single stereo track".
(Note - You may have to mute all the other tracks when you do this to exclude them, I can't recall the specifics and I'm not at my DAW. If it doesn't work properly without muting, then just mute them and do it again. Leave just the track you want to convert to stereo unmuted.)