as Italo already said
try to build an extension for something people need and App Inventor can't do with simple built in blocks.
see also the already available convert number block
http://ai2.appinventor.mit.edu/reference/blocks/math.html#convertnumber
Taifun
PS: please also do not mix your different threads... I moved your last post into the correct thread, which is this one... thank you...