Yes you can use a 12V regulator and feed 12V into the centre pin of the fan voltage select jumper if you want to run all your fans at 12V. However, if you have just a few axial fans then it is easier and cheap to replace them by 24V fans, which are readily available on Amazon, eBay etc. 24V blower fans are a little harder to get but there are some available, e.g. this 51mm one http://uk.rs-online.com/web/p/blowers/7878960/.