Is there a joker caracter to search in q2a sites?
No, there isn't. By the way, in English, it is called a wildcard character instead of joker character. That seems Spanglish to me :)
How to make the researches within q2a more flexible ?
You can implement a search module. Depending on how you are planning to implement the backend of the search, you might need only to implement the process_search function or maybe all of them.
So is there a possibility to implement a joker like a '?' o a '*' to help users to get more results?
As I said before, there is, and you don't need a core developer to do this. But bear in mind there is a tradeoff between performance and the implementation of the feature if you're thinking about the simple/naive approach of implementing this feature in plain SQL. A better approach would involve text indexes, make it the MySQL one or another solution that could implement those indexes (http://en.wikipedia.org/wiki/Lucene).