As Tom Brandt says, you can use the CKERROR intrinsic to extract the byte.
The error numbers are the same as those documented under FCHECK.  You can
call FCHECK if you move the return from CKERROR to a COMP field.

You can also unpack the value yourself with redefines or reference
modification.
E.g. (warning: untested code ahead)

 01 my-file-status pic 99.
 01 redefines my-file-status.
    05 my-file-status-1 pic 9.
    05 my-file-status-2 pic x.

 01 fserr-disp   pic 9(4).

 01 fserr        pic s9(4) comp.
 01 redefines fserr.
    05                pic x.
    05 fserr-lowbyte  pic x.

 01 fserror-msg   pic x(72).


 if my-file-status-1 = 9
   move 0 to fserr
   move my-file-status-2 to fserr-lowbyte
   move spaces to fserror-msg
   call intrinsic "FCHECK" using fserr, fserror-msg
   display "File system error " fserr, ": ", fserr-msg
 else
   ...
 end-if

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