1 - DO NOT HAVE THE GAME RUNNING YET! Did the exclamation mark do it for you too?

2 - Plug in your Xbox 360 Wired controller and leave it plugged in because you're going to set it up, then go straight to playing the game so why bother unplugging it? Unless you already have it plugged in, then you've saved yourself some serious work and can swiftly, but carefully, move your eyes down to the next step in this guide.

NOTE: I'm being serious about leaving that Xbox 360 Wired controller plugged in.

3 - Grab the Xpadder program
It's free: http://games.brothersoft.com/xpadder-download.html

NOTE: Be sure to DECLINE all the junkware it offers you during the installation process.
Oh but make sure you don't rapidly click decline just in case you decline the program it self.

NOTE: If you're using WIndows 7 or later, run that freshly downloaded and junkware denied Xpadder program in Windows XP service pack 3 compatibility mode, otherwise the program displays a 'MUH HURT FEELZ' message and won't cooperate.

LONGER NOTE, THAT'S LIKE A MINI GUIDE WITH STEPS TO FOLLOW SORT OF NOTE:
If you want to know how run to a program in compatability mode:
1 - Right click the Xpadder program icon which opens up a menu
2 - Click on Properties
3 - Click on the Compatibility tab
4 - Click on the box next to 'Run this program in compatibility mode for:' which will enable the drop down menu below
5 - Click on the drop down menu to open it up
6 - Click on Windows XP (service pack 3)
7 - Click Apply
8 - Click Ok
9 - Stop clicking stuff for now, don't even click your fingers in case you upset the balance of the universe with all that clicking.

4 - Snatch this Xbox 360 controller image right off those internets like a pro
It's also free: http://images.akamai.steamusercontent.com/ugc/576732510392225574/441D2AD321301236DBD10D119C9FA61B5F7FA8CA/

NOTE: As soon as the image has downloaded, open the image in MS paint, or whatever photo editing program you use and then simply save it as .BMP and it's best to save the image in the same folder as Xpadder is in to keep them together.

5 - Sneak a quick peek at this tutorial to show you how to load the image into Xpadder
Not a single penny required: https://www.youtube.com/watch?v=rSSLvFpyd7g

NOTE: Skip to 1:20 to see the part about how to load the image into Xpadder and continue watching to see how to set up the basics.

NOTE: If you want the Right analog stick for rolling your spaceship left and right, then click on the Left box for the right stick and select Q (This is 'Roll Left' in game) Now click on the Right box you select W (This is 'Roll right' in game)

OPTIONAL BUT LIKELY TO BE WORTH IT: Eye up this video for an 'advanced tutorial' on how to use Xpadder if you need to
No coins due here: https://www.youtube.com/watch?v=hBrM0U2VsTk

6 - Gaze at this short guide to help with front end settings for Xpadder and X3: Reunion
This is also free if you can believe that: http://forum.egosoft.com/viewtopic.php?p=2821823#2821823

7 and LAST STEP - Now with all your newly grabbed knowledge,controller still having never been unplugged at any time during this guide, Xpadder still running, all coins still in your purse, load that game up and do a barrel roll. If not for me, then do it for the poor people out there who won't find this guide and never get to barrel roll using a control pad in this game, it doesn't cost anything.

NOTE: here's another exclamation mark because i just know you enjoyed the first one !
... twas good wasn't it.

Looks like the end of the guide, but it isn't really as there still some more words down there to be read... go see for yourself

A big THANKS goes to the guy at the forum, without his help I'd never have got to do a barrel roll with that right analog stick!

Another big THANKS to those voices behind the making of those Xpadder tutorial videos, without whom i could never have dreamed of being bothered enough to grab that image right off the internets to use in Xpadder or to learn more than i really needed to learn about the Xpadder program.

Yet Another big THANKS to whom ever it was that made that xbox 360 control pad image, it's ok that it wasn't already in .BMP format, we'll just do some clicks on some stuff and make it happen.

None of those mentioned above know it, but they've helped me and trillions of other space cadets on Steam do barrel rolls with the right analog stick on our wired Xbox 360 controllers in this game.