[defcon1_es]

Hola. Varias cosas que yo tendría en cuenta:

1) Yo pondría en los uses a la unit ShareMem

Código Delphi [-]
unit Conversiones;  

interface  

uses Sharemem, ib_util, SysUtils, Classes;

Código Delphi [-]
library g_udf;

{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }

uses
  Sharemem,
  SysUtils,
  Classes,
  Conversiones in 'Conversiones.pas',
  ib_util in 'ib_util.pas';

{$R *.res}
  exports    
    NumeroALetra;

begin

end.

2) El fichero g_udf.dll debe estar en la carpeta UDF de Firebird

3) Borra la definición de la función de tu base de datos y vuelve a darla de alta tal como sugiere jhonny