I need to convert a Particle System to a mesh object, so I can export it to Unity 3D. How could I do this?
The following assumes that you are using 'Particle System-> Render-> Object -> Dupli Object'.
Option 1) You can press the Convert button for the Particles Modifier.
Option 2) If you would like to keep the particle Emitter intact, you can use the 'Make Duplicates Real' Tool Shift+Ctrl+a.
This will create object particles at every particle position just as if you applied the Modifier but the particle emitter and its particle system will still be intact in case you would like to make further changes.
This is pretty easy to do for hair particles. In Blender, particles are "stored" in a Modifier, which means it can be applied like any other; This only works for hair particles, for obvious reasons.
To convert your Particles, go to the Properties editor and navigate to the Modifiers tab; Here, you'll find all modifiers applied to the selected object, including the Particle System modifier. You can then press
Convert to convert your particles into a mesh.