I have heard that its possible with some mods. Does anyone know what kind of mods? Like drilling new holes for foot pegs or bending the nerfbar bracket? Thanks!

 

Probably depends on what brand of nerf bars since different brands mount differently on the 400EX

 

I got a set of DG nerf bars

 

Not sure how those mount but on the 400ex
Some nerfs use the foot pegs in the rer and bolt to the frame in the front, others (like ProArmor) bolt to the foot pegs and use the lower A-Arm mounts in the front.
The only way to know if they'd fit a 300ex is to try.