Found the solution but don't know if it's nice:
ffmpeg -i a5732-480x320-clip01-sound.mp4 -filter_complex "[0:0]split[main][back];[back]scale=1920:1080[scale];[scale]drawbox=x=0:y=0:w=1920:h=1080:color=black:t=1000[draw];[main]scale='if(gt(a,16/9),1920,-1)':'if(gt(a,16/9),-1,1080)'[proc];[draw][proc]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2[fhd];[fhd]setsar=1/1[sarfix]" output.mp4
Details: split video stream to two streams. First one I scale to 1920x1080 and then fill all area to black color. Second stream I convert to 1920x1080 saving aspect ration so video is always less or equals FullHD resolution. Then Second stream overlays the total black stream so free areas becomes black. And last filter is fixed SAR.
Maybe someone know more elegant solution?