On the timeline, no. Once an item is on a timeline anywhere in your project, you cannot alter the amount of audio channels, only how they are mapped.
Best thing to do, as I assume you want to add some effect that only works in stereo is go to the Audio mixer panel, create a stereo submix and send your mono channel to that. You can then apply stereo effects to the stereo submix on the mixing panel.
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Angelo Lorenzo
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