You can make a mod that removes the zoom from the guns
Go to:
Ghost Recon - Desert Siege/ Ghost Recon Data/ Mods/ (Origmiss)/ Equip/ then edit the .gun files
change the line(s) that say eg: (oicw)
<ZoomSettings>
<Zoom>1</Zoom>
<Zoom>5</Zoom>
to
<ZoomSettings>
<Zoom>1</Zoom>
<Zoom>1</Zoom>
Then save the edited file as a new file.. . Then erhm.. . never acctually made a mod myself so this is as long as i know
But i think you need to make a new .kit file and link the "new guns" you made to that kit.
You however not add iron sights to the game as in a 1st person weapon.
Hope that helped some