What will happen if the 6th gear is put in a 150cc bike?

There won't be any additional pick-up and its performance differences nor fuel consumption from the Pulsar150 dts-i model2004 if it is upgraded to sixth gear.

There were already couple of incident during 2004 to 2007 on few Pulsar version and it was not because of Pulsar bikes but few young riders who went beyond the control of it while riding them.

There are only few racing bikes that has sixth gear other than like Suzuki GS150R and Honda Dazzler, at the moment. Sixth gear bikes might help for pickup but mileage will be less unless bike is less weight comparison. The more sixth gear bike for common people may release in the future.

Most of the time 150CC bikes don't need to 6th gear due to low power. In simple words the power will be divided in 6 gear which will result in less power to gear ratio. There are other side effects of this as well such as engine straining, gearbox damages faster, fuel efficiency issues, etc. Manufacturer have hundreds of engineers who expertise in this field. If it would be beneficial then they would have been launched in every new bike.

I think adding the sixth gear would be impractical decision. Most of the time addition of new gear is done for performance and pick up improvements. Bikes above 100 CC are already hitting the limit with the pick up and speed. So adding one more gear for bikes above 150CC has to be compensated with safety too. So far there are no valid reason to manage the additional gears. I'd say no automotive company is going to introduce the additional gear.