In my opinion, a good underwriter can be taught the mortgage rules IF they have the basic smarts and analytical talents. It's not an entry level position and most often underwriters move up out of mortgage processing. I have to say, however, that most of the underwriters I hired and trained when I was in mortgage banking did not. Most were simply excellent performers in a related department.
I look for and test for the same qualities in my title insurance agency staff now. I want a person who can read and comprehend instructions and guidelines, analyze and resolve problems, maintain quality of product and service, work efficiently and has a good attitude.
Any position in the mortgage or title insurance business requires long training at the side of an expert. So my advice is to find a good underwriter under which to learn your craft and then demonstrate your abilities by doing an excellent job. You will be noticed. Once you have achieved credibility with management based upon your quality of work, let them know you'd like to move into underwriting.
In the meantime, read and study all available resources - there are lots of good web sites - to learn MORE than you are taught in your office. You will be noticed. ;)