Bueno he logrado abrir el archivo en forma del tipo APPEND
aqui va el codigo:
Public Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type
Public Const FILE_APPEND_DATA As Long = &H4
Public Const FILE_END = 2
Public Const OPEN_ALWAYS = 4
Public Const FILE_ATTRIBUTE_NORMAL = &H80
Public Const INVALID_HANDLE_VALUE = -1
Public Declare Function SetFilePointer Lib "kernel32" (ByVal hFile As Long, ByVal lDistanceToMove As Long, lpDistanceToMoveHigh As Long, ByVal dwMoveMethod As Long) As Long
Public Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As SECURITY_ATTRIBUTES, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Codigo:
dim HandlerFile as long
dim sa as SECURITY_ATTRIBUTES
handlerfile=createfile("c:\pepe.txt", FILE_APPEND_DATA,0,sa,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,0)
if handlerfile<>INVALID_HANDLE_VALUE then
setfilepointer(handlerfile,0,0,FILE_END)
'Aqui con la llamada a la api writefile funciona correctamente
end if