Using questions from a code-competition website in homework and exams
Whether this is a good plan or not depends on some things, most especially the nature of your students. 

However, it seems a bit too unstructured. If you have a general mix of student ability it could cause problems unless you incorporate the questions from the site(s) more formally into your teaching. There are a lot of questions available, I would guess and some of your students will panic, thinking that (a) they need to do everything or (b) it is hopeless. This is a special problem if these extra questions are above and beyond all necessary assigned work. 

However, one good aspect of your plan is to show them in advance the kinds of things that they will be expected to know and that other students also regularly deal with. Much better that than hiding your intent and springing things on them unexpectedly.

But I think a better plan is for you to incorporate a sampling of the found questions into your labs and homework assignments. For the most challenging questions you could have five minute brainstorming sessions as part of several class periods. 

In short, as part of the regular process it should work out, but if it is completely "extra but not formally rewarded" work it might backfire on you. 



An additional dimension here, of course, is that you could use the site(s) to teach yourself how to ask better questions. And your focus on "creative questions" rather than necessarily "challenging questions" is a good one, I think.