I've done it on our beans for the last few years.  I raise seed beans, and the seed company encourages us to do this, as the beans crack less if they are higher in moisture.  Do a google search for "Soybean Equilibrium Moisture Chart" and it will pop right up telling you what temperature and humidity you should run your fans to add moisture to the bin.  Some things to be careful of:

If the bin is clear full and you add moisture, I have heard it can crack the bin.  I've never seen it happen, but have been told it can.  
Run the fan on a drier day when you are done adding moisture, as the bottom beans will probably be wetter and can stick to the floor.  They can still be taken out, but it's a pain.  I try to run the fan for a half day on a dry day to make my life easier taking them out.

I usually only put about 2 points of moisture in, I'm sure you can do more, but I've been happy with that.  It adds up to thousands of dollars if you have a decent sized bin and do the math.

It's a rainy day today, and I have the fan on my bean bin right now adding moisture to beans.