I know a little about the dextool and apktool which can be used to decompile the apk. but neither of them explicitly explain adding any new activity to the decompiled APK. is it possible, and if its can you give me an example or any link directs me to explanation? thanks
Firstly do not use this in a sinister way, we've got enough people doing that already. The decompiled app can be added to eclipse by importing from existing source. Within this you can add a new class and then export a signed package (signing it with a new key).
Step by step can be found here.