How can I elongate arm muscle?

This is a very common misconception. Drumming has nothing to do with strong arms and drumming does not build arm muscles. Drumming is about finesse and actually drummers should not lift weights at all for that very reason. All of this aside, someone has given you some very false information. Without the presence of testosterone you will not get "beefy" arms even if you lifted weights hours a day, and no amount of drumming is going to do this. Female bodybuilders take testosterone injections and that's why they bulk up. You have absolutely nothing to worry about.