As a person working in IT, I can tell you you'd best provide links to the tool and probably also its source code if you want someone to be able to edit it or add to it. Plus some example games/obk files with and without underpromotion.
I'm not saying I will btw 
I don't have the source code of obk2pgn, but I have the source code of obk2bin
obk2bin douse something similar as obk2pgn, but obk2pgn is more accurate
Obk2bin is included with the download OpenBook http://goneill.co.nz/chess.php
The source codes are included in this download.
obk2pgn can be downloaded here: https://web.archive.org/web/20181129180105/http://homepages.paradise.net.nz/npattins/ChessOpeningBookPage.html
I have a 64 bit version of obk2pgn it works the same as the 32 bit version only faster.
I just find out that the problem is not with obk2pgn but with the MakeBook tool.
The problem is with the created obk books. There are no underpromotions in the obk books (the pawn always promote to a Queen), even if the obk books are created with pgn games with underpromotions. The same problem when I create the obk books with Chessmaster 9000
Hello, I am a programmer and i am working on .obk files to make a conversion program (from Arena .abk format).
I have read the format specs and tested with the editor included in Chessmaster XI, and I can assure you that minor promotions are not supported. Only two bytes are used to store the moves, so these are just the basics: the starting square, the finishing one and the priority with which it is played. That's all.
Thanks for your reply.
I find conversion from obk to pgn with obk2pgn useful to filter out the main moves of a database for white or for black. You know when you make a obk book with Chessmaster 9000 there is an option to add 3 dots to the most played moves, 2 dots to moves the are also played frequently, 1 dot to moves that are not played that often, and and no dot's for rare lines.
After the conversion to pgn you can remove the rare lines by removing the lines with a '?' in the pgn notation.
I also have a special tool to make obk books where I can specify how much percentage a move have to be played, to get 3, 2, 1 or no dot's when making a obk book.
Like you also find out there is a limition when there are lines with underpromotion in the obk books.
Can someone help with this.
I have a tool obk2pgn that can convert obk books (Chessmaster 9000), to pgn with informant symbols.
obk2pgn works great as long there are no lines with underpromotion in the obk books.
If there are lines with unerpromotion in the obk books obk2pgn fails to convert the obk book to pgn.
If someone thinks he can help me with this please give me you email address, so we can discuss it.