Just asking if it is possible. My friend added gears, but won't it damage the look or my cycle?
It is possible. There are a lot of 20in wheel bikes with gears. There are some problems, though:
The least of your problems would be "looks"...
You can throw some more dollars and install an INTERNALLY GEARED HUB, which not just wouldn't break the clean looks, but would actually benefit from the horizontal dropouts found in BMX frames. But those are usually quite expensive.