Thanks for the feedback. Sprite sheets are not supported at the moment but this is at the top of our features for the next version. Currently most of the platforms that Particle Designer exports for don't support a single system with multiple emitters. For this reason you can export a stage file from PD which allows you to read in details of where multiple emitters should be placed to work together. Admittedly this is a file that would need to be read manually and then used to manually place the emitters together.
We are reviewing the different particle systems in frameworks at the moment and reaching out to see if the framework owners/maintainers would be willing to make changes to the particle systems to better support this ability.
With the ability to export to a sprite sheet, this would mean that no extra support is needed and therefore the timeline approach in PD would allow for some cool effects to then be exported to a sprite sheet. This is also on the list
We are prototyping ideas and changes at the moment for the next version. I don't have a time frame just yet but we want it to be ASAP.
Hope that helps and thanks again for the feedback.