In general $W/m^2$ and $W/sr$ are different units of measure for radiant energy so can not be converted between.
However, given the geometry of the area to be measured and the radiant energy for that area then conversion can be done:
To convert from watts per square meter (W/m²) to watts per steradian (W/sr), you would need to determine the solid angle subtended by the surface. The solid angle can be calculated from the surface area and the distance from the surface to the point at which the radiation is being measured.
Once you have the solid angle, you can convert W/m² to W/sr by dividing by the surface area:
$W/sr = (W/m²) / (surface area / (distance)^2)$
where surface area is expressed in square meters and distance is expressed in meters.
Note that the resulting unit will depend on the surface area and distance, and the formula is only valid for isotropic radiators, i.e., radiators that emit energy uniformly in all directions.