I know I'm posting an answer to this WAY after the fact, but I was curious about this as well about a year and a half ago and I have discovered that it is possible. I figured I would post a response to this for educational purposes.
The main thing that I learned to do that has made me into a precummer (whereas before I rarely got any) was edging. When I first started edging, that is masturbating until you get close and then stopping, I noticed I would get a little bit of precum. As I learned to edge for longer and longer periods of time, I noticed that the amount of precum that I would make increased dramatically. By learning to stop an orgasm before it starts, and by building my
stamina so that I could edge for a few hours at a time, I conditioned my cowper's gland to produce more of the sticky substance. I now can have multiple small orgasms that usually result in streams of precum that spurt out. The amount of preseminal fluid that I can produce now without much effort still amazes me at times.
Why would anyone want to learn this? It's fun and I now know a lot about my body that I would have never learned if I hadn't started edging. My orgasm control is phenomenal. And I have also noticed that I cum a lot more too since practicing edging. I've always been a big cummer, but now it's ridiculous.
I thought maybe my stumbling onto this was an isolated incident, but I have had a few people ask about it in reference to my xtube videos, and my advice has also helped them produce more precum and become multiorgasmic. If you are interested in learning more about edging, a basic tutorial can be found at this link:
http://www.orgasmedging.com/edging.htm
I hope someone finds this useful!