If you still only have a 5GB Box account, get an upgrade to 15GB for free by signing in to Box using Astro file manager on your Android device.
This was notified in Astro's Facebook page on 8 January:
https://www.facebook.com/ASTROFileManager
And their Twitter on 7 January:
https://twitter.com/metagoinc
Astro is a file manager (like Windows Explorer is) for Android phones and tablets.
Will it stack with my 50GB ?
Answering myself, no.