I rendered color, depth and motion(mt) channels, but I forget to add 2D Motion Blur output shader. Is it possible to do 2d motion blur effect without rendering these frames again?

you could use a motion estimate/ motion blur plugin in post. like the rsmb for afx. sometimes it even works quiet well without a rendered motion vector matte.

also, rerendering motion vectors (using lm2dmv) only should not last too long unless you don’t have millions of polys in your scene.
instead of using channels you could assign the motion vector material to a pass partition. it renders faster because it doesn’t have to render the other channels again.

Thanks for tips have huge scene with lots of stuff :(
I stillwomder is there any way to use .mt motion files? If not what´s the point of storing those anyway if there is no use for them. I even tried rerender but SI didnt use the old mtfiles?