First of all, I'd say the basic idea that in German, you can form adjectives ad-hoc by attaching a suffix is right. However, it doesn't quite work the way you outline.
Let's go through your question sequentially, also including some tangential remarks:
As is already explained in the post you found, there are several common suffixes that can be attached to ...
Well, the sentence
Hans stiftisch schreibt.
is completely wrong. No native speaker would even think of constructing such a sentence. Even worse: Nobody would understand it in the way you want it to be understood. It is wrong for two reasons:
There is no adjective "stiftisch". Even if you would use it grammatically correct (see next item), nobody ...