I’ve readed about some request on this but nothing done yet.
I’ve found some aproaches:
· Apply it on a material and use lightmass to calc the light but the quality seems not to bee so good.
· Light volumes but they seem to be stop developed so is not a good path to take.
· Focus a light on a plane, that’s ok but the light shape coming from a cone is a circle, anyway not too bad option.
· Make a grid of cylindrical lights, good one but you’ll get areas with more light than others and they emit for both sides.
· There are some post from 2014 speaking about a future implementation of basic shape based lights but no updated.
Obviously my intention is to have anykind of area light as is done in V-Ray, Mental Ray, Thea,Corona, Maxwell etc.
Any other idea?
There are only sphere and tube lights supported right now…Epic doesnt have an implementation for rectangular light sources. So well…you do get area lights, but not all of them 
I think there is not really a proper way to fake them especially if you want rectangular specular as well. Only thing you can do to emulate the effect is to use an emissive box/plane shape, place a wide angled spot in front of it that is the actual light source and put a reflection capture next to it so that everything getting close gets the captured reflection of the emissive box/plane. But overall, there is not really a proper solution to this for now.
EDIT: If you are using lightmass, you can also enable the light source angle which will help you to get softer shadow penumbra to even further emulate softer light coming from a box shaped source
It’s a shame we cannot get a proper solution for rectangle lights since they are one of the most used lights in arch viz.