I have –simplified– the following:
\starttext

\defineframed
  [action]
  [width=5cm, height=3cm, offset=none, frameoffset=.5\linewidth]

\action{}

\stoptext

What I would like to do is to let the user input some text and display it in
this frame. (In reality there will be more frames.) When the text is to big
to be displayed in the frame, it should be rejected.

This is so that the document can be modified with the text from the user.

I also would like to save the input from the user in a file, so that it can
be used later again.

Is this possible?

-- 
Cecil Westerhof