i have an aluminium frame single speed. I wanted to know is it possible to add gears to it (correctly). I saw same model with gears installed but the rear wheel was not in line with front wheel and was moved to a little left to accommodate the 7 speed gear freewheel. Is this something acceptable or a disaster to happen.
The rear spacing on most single speed frames is 120mm. The dropouts usually face backwards and can't take a derailleur very easily. Is your bike like this?
If so, a hub gear may be the best option. You can get a 2, 3, 5 or 8 speed hub gear that fits a 120mm rear end. This will maintain the clean look of a single speed, especially the 2-speed, that needs no extra cables.
If you try derailleur gears, you will need to create a hub setup that has enough space on one side for the sprockets/freewheel. You are unlikely to be able to do better than 5 gears, and mounting the derailleur will be a challenge unless you find an adaptor fit for purpose.
As JoeK said the one of the biggest problems converting a single-speed to a derailleur setup is unsuitable dropouts and a lack of a derailleur hanger. Also, lack of frame stops for shifter cables is an issue. However, seeing as you say up have seen bike of the same model converted, this must be possible.
If you have 120mm dropout spacing rather than 130 or 135mm now standard on road bikes you'll need a freewheel type hub with a lower number of gears rather than a freehub that accepts a cassette and you'll be limited in the number of sprockets.
BTW, hubs for freehbs or cassettes do have the spoke flanges offset to the left, but the spokes are 'dished' so the rim is centered in the frame.
If you have slotted dropouts you'll need a hub with a threaded axle fixed with nuts. These do a much better job of keeping the hub fixed in the dropouts than a quick release.
If your frame lacks cable stops, there are clamp-on stops available, or you run a continuous length of housing an zip-tie it to the frame.