For a 108" diagonal screen size you need to have at least 12.5 feet from the lens to the screen. There is no way to make the 108" diagonal without at least that 12.5' distance.
Check here: http://www.projectorcentral.com/BenQ-W1000+-projection-calculator-pro.htm
If you don't have at least 12.5 feet to work with, then you may want to try a mirror bounce, which can be tricky to setup. That is, point the projector at a first surface mirror, or mylar mirror and bounce it at the screen. Point the projector at the BACK of the room towards the mirror and the mirror towards the screen. This is tricky to set up, but it effectively can increase you range to what you need it to be. So, if your room is only 12 feet deep (about 11 feet from the lens to the screen) you turn the projector backwards and move it a foot from the mirror. The mirror is now nearly on the back wall and the projector is a foot from the wall. That means you total throw distance would be about 13 feet, which would fill a 108" diagonal.
There are no effective add-on lenses available for projectors. It would be cheaper to buy a new projector than get a proper add-on lens.
This is one of the reasons BenQ switched to a shorter throw distance with their newer entry level models. The W1070, then the HT2050 and HT1070 both can hit a 108" diagonal from about 9 feet away instead of 12.5 feet.
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