Particles in combustion are acutually bitmap shapes.
What you need to do is create your shape in combustion/photoshop. It has to have dimensions of 128x128 pixels.
Once you have created your shape, go into combustion and apply the particles operator to the layer. Find the text particle you would like to use and place an emitter on the viewport so you see the particles on the layer.
In the workspace panel you should see the particle operator with a sub-object (emitter). If you expand the emitter, you will see the particle type. Usually for text effects, it is called “word”.
Now in the particles menu, select the SHAPE tab. Click import and you will navigate the the directory where your shape is. When the shape is selected you must click OK then the next pop-up window, you will set the setting for the file, such as alpha settings etc. Click OK when you are done.
You should now see your shape in the shape preview window. Select your particle type (“Word”) in the workspace panel and click the SWAP button. This will change the shape of the particles.
You then just need to go through all the behaviour settings and adjust them accordingly.