[FFmpeg-user] Can FFMpeg insert a video or audio into another video or audio?

[大山]

To whom it may be concerned:
Hi,
I'm a user of ffmpeg. I know ffmpeg can concat two videos or audios into a new single media file, but I want to insert a video or audio into the middle of the media file,for example, the duration of "a.avi " is 00:05:00, and the duration of "b.rmvb " is 00:01:00, I want to insert "b.rmvb" into the postion "00:02:00" of a.avi, the duration of new a.avi is "00:06:00",how to fulfill it? if not, can you add this new function into ffmpeg and update it?
 thanks for your attention.