Is it really necessary to download an alternative SDK for this, or, in theory, I could built it by myself without any extra-download?
Wikitude and similar frameworks were written by programmers. Other programmers could write one as well. Hence, in theory, if you are a programmer, you could write one.
However, since something like Wikitude probably has in excess of a hundred developer-years worth of engineering in it by this point, you may wish to use an existing framework, unless you have a rather long timeframe for developing the app.