Creating custom attributes in Active directory is a pretty straight forward thing. There are numerous articles on this. Links to some good articles are given below -

http://www.awprofessional.com/articles/article.asp?p=169630&seqNum=1&rl=1

Once the attribute has been created the only task left is to create a new property under User Profiles and Properties and Click on Add New property. Finally map this property to the new attribute you added as per the article above and run the profile import.