If I understand you correctly, you want the file field description copied and written to a file.
In a CL do a DSPFFD to an outfile and then call a pgm (RPG etc) read in that file extracting the description as required.
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SELECT dbilib, dbifil, dbifmt, dbifld, dbitxt, dbihdg
WHERE dbilib = '[myownlib]' and dbifil = '[myownfile]'