Hi Sabine,
You can do this with Mif2Go. MIF is not listed as output format on the web-site,
but nevertheless MIF is one of the output options. And Mif2Go is free.
The link is:
Mif2Go converts FrameMaker files to HTML/XML, Help, and Word | mif2go.com
(I do not know why the URL is converted to a descriptive tex automatically; the same below.)
Or the i-frame translation pack (works only in combination with FrameScript):
itl – FrameScript Portal: Convert: FM to MIF / MIF to FM
You might also adapt ExtendScript samples which you can find somewhere, e.g.:
Sample Scripts/Utilities shipped with FrameMaker10 « TechComm Central by Adobe
Best regards
Winfried