There is an invention set called Sudden Acceleration that has a special enhancement that, when slotted into a power with knockback, turns that knockback into knockdown. This enhancement is not unique and is of relatively reasonable cost to buy/craft. You could conceivably slot this enhancement in any/every power you have that has knockback thus turning them all into knockdown powers instead.
There is also the Overwhelming Force set from the Summer Blockbuster event. This set can also be purchased from the Merit vendor. The pieces of this set are all unique so only one of each piece can be slotted. One of these enhancements in this set turns knockback to knockdown if the power has knockback OR adds a chance to knockdown if the power does not have knockback. This set is expensive.
Great ! Thanks a lot for this, was exactly what I was looking for =)