It's possible, and there are a few techniques to accomplish it, but some strains will resistant to 'natural' methods.
The first thing you need is 'female' pollen. You can try letting your girl flower but don't harvest all of the bud. some strains if left alive will begin to grow pollen sacs which you can then harvest for pollen.
Doing 'stressful stuff' to your girl during flowering can cause her to produce pollen sacs. Irratic light patterns, poor nutrition, and physical damage could all cause undue stress that might trigger a 'survival instinct'.
I have created feminized seeds with the above techniques, but some strains just won't make sacs for me.
Gibberellic Acid, STS, colloidal silver, and a few other chemicals can cause your plant to produce pollen sacs as well, but i have not played with that yet.
