Prototype the OVRDBF or any command to be called via QCMDEXC method.

Define Prototype for ILE call to QCMDEXC.  Of course if you don't 
want to get into ILE programming, you can just define the CMD fields and use 
the OPM CALL 'QCMDEXC' with PARM opcodes. e.g.
d Qcmdexc         pr                  Extpgm('QCMDEXC')                 
d  Cmd                                Like(Qcmd) Options(*Varsize) 
Const
d  CmdLen                       15p 5 Const                             
d  CmdOpt                        3    Options(*NoPass) Const            
d Qcmd            s           3000
- - Define the command or portions thereof as constants as needed.
e.g.
 *-----------------------------------------------------------------------
 *
 * Named Constants                                                         
 *-----------------------------------------------------------------------
 *
d CrtDupP1        c                   'CRTDUPOBJ OBJ('                     
d CrtDupP2        c                   ') FROMLIB('                         
d CrtDupP3        c                   ') OBJTYPE(*FILE) TOLIB(QTEMP)'      
d OvrPrtf         c                   'OVRPRTF FILE(QSYSPRT) PAGESIZE(*N 1-
d                                     98) CPI(15) MAXRCDS(*NOMAX) OVRSCOPE-
d                                     (*JOB) SHARE(*YES)'                  
- - Assign command to variable

e.g. ILE
c                   Eval      Qcmd = CrtDupP1 + 'RPSRVCWK1'      
c                                  + CrtDupP2 + %Trimr(DtaLib)
c                                  + CrtDupP3                    
c                   CallP(e)  Qcmdexc(Qcmd : %Len(%Trim(Qcmd)))  
c                   If        %Error                             
                       :
c                   EndIf

e.g. RPGLE (OPM)
c                   Eval      QcmdLen = %Len(%Trim(Qcmd))
c                   Call(e)   'QCMDEXC'                  
c                   Parm                    Qcmd         
c                   Parm                    QcmdLen      

I hope this helps you dig in.....
As for the conversion of the source to RPGIV, I've encountered a minor
problem when an external DS is defined and an array by the same name
redefines a sub-field of the DS.  I've had to rename the array field
throughout the program and define it within the DS using the OVERLAY
Keyword.

e.g.
RPG
E                    DXF         9  2
IPIQ000    E DSIQ000DS

DXF is defined in the external DS as;
 FMT J  .....I....................................PFromTo++DField
                                                   66956712 DXF
RPGLE
d Piq000        E Ds                  Extname(Iq000Ds)
d  DxfA                          2    Dim(9)   Overlay(Piq000:6695)

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