Pretty much every video converter nowadays can be used convert h264 video data to Adobe Flash video files. H.264 codec is standard for Adobe Flash container and h264 video data can be integrated to FLV file without any quality loss or complex re-encoding.
Open H264 file Open FLV fileA video converting tool for macOS / OS X
One of the many multimedia converter programs for Windows
Easy to use video converter for Mac
Video converting software