I belong to a very large HS association and there is one person who literally picked up umpiring after long break and within five years has worked a nearly full D2 schedule with the only breaks being eight varsity games so he is tournament eligible. He's in his 50s and just went down to Florida for his first time to work the winter collegiate games. He networked like crazy and went to any camp he could in the Midwest. He'll be D1 within a couple years and the only thing that sucks is that he won't have that long because of his age and the break he took. But it's absolutely possible to get there without umpire school. Basically it's network, be available and go to camps. Of course being a good umpire is a pre-requisite, but if you weren't one now, then you probably wouldn't be asking the question.