except that haddock expects the new PDB format for nucleotides which has three letter notations like ADE, GUA, CYT, THY.
Well, if that's indeed the case, HADDOCK is 
not using the new PDB format as documented in "
Remediation of the protein data bank archive". Specifically, DNA residues now should be named DA, DC, DG, and DT, as shown in an example (
355d) below:
ATOM     26  C2'  DG A   2      22.447  27.195  19.590  1.00 10.31           C  
ATOM     27  C1'  DG A   2      21.722  26.527  20.744  1.00  8.31           C  
ATOM     28  N9   DG A   2      20.293  26.737  20.884  1.00  6.86           N  
ATOM     29  C8   DG A   2      19.536  27.799  20.464  1.00  7.02           C  
3DNA generated nucleic acid structures (including 
fiber) do not follow 
strictly the PDB guideline, which seems having not caused any practical problems. As a rule, I do not tailor core 3DNA to any specific third-party tool. For background information, please see my blog post "
PDB format, how many variants are there?". 
That said, you may easily write up a format-converting script to fit your needs. In 3DNA v2.1beta, the 
perl_scripts/ directory contains a 
simple script named 
x3dna2charmm_pdb that converts DNA residue names from one-letter to three. It is enclosed below for reference, and you are welcome to customize it (especially the 
two lines in red). 
Let us know how it goes.
Xiang-Jun
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#!/usr/bin/env perl
use strict;
use warnings;
## This is utility Perl script for converting 3DNA generated PDB file
## to that accepted by CHARMM. Initially written in response to a
## request from a 3DNA user.
## Please note that this script may not be that sophisticated, since I
## know little about the specifications of the CHARMM PDB format.
## Please let me know if you find any bug in it.
die "Usage: $0  3DNA_generated_PDB  converted_PDB\n" unless @ARGV == 2;
my $x3dna_pdb  = $ARGV[0];
my $charmm_pdb = $ARGV[1];
open( FH, "$x3dna_pdb" )   || die "Can't open <$x3dna_pdb> for reading: $!\n";
open( FO, ">$charmm_pdb" ) || die "Can't open <$charmm_pdb> for writing: $!\n";
while (<FH>) {
    if (/^ATOM/) {
        chomp;
        # expand this list as necessary ...
        my %one2three = (
                          '  A' => 'ADE',
                          '  C' => 'CYT',
                          '  G' => 'GUA',
                          '  T' => 'THY'
                        );
        my $residue = substr( $_, 17, 3 );
        substr( $_, 17, 3 ) = $one2three{$residue}
            if ( exists $one2three{$residue} );
        my $chainID = substr( $_, 21, 1 );
        substr( $_, 21, 1 )  = ' ';
        substr( $_, 54, 18 ) = '  0.00  0.00      ';
        substr( $_, 72, 1 )  = $chainID;
        print FO "$_\n";
    } else {
        print FO;
    }
}
close(FH);
close(FO);
=for example
Sample CHARMM PDB file:
ATOM     27  H2' ADE     1      -3.643   6.134   3.867  0.00  0.00      A
ATOM     28  C3' ADE     1      -4.155   7.022   5.806  0.00  0.00      A
ATOM     29  H3' ADE     1      -5.203   7.382   5.738  0.00  0.00      A
ATOM     30  O3' ADE     1      -3.256   8.063   5.448  0.00  0.00      A
ATOM     31  P   THY     2      -3.070   8.432   3.902  0.00  0.00      A
ATOM     32  O1P THY     2      -4.195   7.871   3.120  0.00  0.00      A
ATOM     33  O2P THY     2      -2.853   9.889   3.761  0.00  0.00      A
ATOM     34  O5' THY     2      -1.722   7.646   3.549  0.00  0.00      A
Sample 3DNA generated PDB file:
ATOM     50  O3'   C A   3      -7.224  -1.903   7.585
ATOM     51  C2'   C A   3      -6.408   0.308   8.023
ATOM     52  C1'   C A   3      -5.322   0.045   6.986
ATOM     53  N1    C A   3      -4.202   0.995   7.051
Transformed file:
ATOM     50  O3' CYT     3      -7.224  -1.903   7.585  0.00  0.00      A
ATOM     51  C2' CYT     3      -6.408   0.308   8.023  0.00  0.00      A
ATOM     52  C1' CYT     3      -5.322   0.045   6.986  0.00  0.00      A
ATOM     53  N1  CYT     3      -4.202   0.995   7.051  0.00  0.00      A
=cut
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