Many video converters should support both h264 and h261 formats, so h264 to h261 conversion is certainly possible.
For example you can use the multi-platform FFmpeg encoder and command line tool to convert H.264 video data to H.261 video format. Install FFmpeg and type to command line:
ffmpeg -i /path/to/your/file.h264 -vcodec copy -acodec copy /path/to/your/newfile.h261
Transformation H.264 videos to H.261 format decreases video and audio quality.