I have a 10mw blue/violet laser pointer and want it to be more powerful. My friend did it by adding two laser pointers together to hold more batteries to his 30mw green laser pointer. I don’t have another laser pointer to do so, so is there any other way i can add more batteries?

You’d have to create your own battery holder to hold additional batteries and wire it into your existing pointer. As for increasing the actual mw (milli-watt) power of your pointer, it might increase the output, but you also might burn it out if it isn’t designed to handle the additional voltage. Usually the output of the laser is fixed, and increasing the voltage may not change anything, but any modifications to the laser pointer that increases the output can be dangerous and illegal if it exceeds a certain amount or exceeds the manufacturer’s specifications, and any damage caused to other people or objects by tampering with a device like that can be a very serious problem that you may not want to find yourself in.