Easy as pie. Download the bonus merchants from the internet and put them in your override folder after unzipping them.
BTW, I played through the game without the extra's the first time too, and I think I liked it better that way. At that time, Beholders were a real challenge. Now it's just: buy sield, walk to beholder, beholder kills itself.
The other items are also unbalancing the game in my opinion (for example, there's a helm that allows you to cast simulacrum. A double of a tank which can fight just as well and for whose life you needn't fear is a bit too good...).
So, you should be lucky you got a chance to play the game whithout them
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The last arrow of Legolas kindled in the air as it flew,
and plunged burning into the heart of a great wolf-chieftain.
All the others fled. -J.R.R. Tolkien, The Lord of the Rings