Starting with normal 1920/1080 video,  I want to convert it to the vertical aspect ratio seen in cellphones.

A vertical "pillar" black matte is not sufficient. I need the reframed video to play full screen in a vertical cellphone screen.

I need a way to convert a widescreen original to play full screen vertically on a cellphone 

I understand this would only use approx a third of the original video.

Is this doable ?

If you have Studio 24 or 25, you can work with and export portrait videos directly. When adding a landscape video clip to a portrait project you may need to play with the clip's scaling (fit/fill) or use a tool such as pan & zoom to get the video to fill the full height and also keep the subject in-shot (especially if it drifts a little off-centre).

Earlier versions can't work directly with portrait video - however you might be able to do a pillarboxed landscape video and use a tool such as HandBrake to crop it and turn it into a portrait video afterwards

Regards,
Richard

good advice.

And very interesting that the newest versions include that export mode

 thanks