I was trying to create a realistic explosion within the engine but was kind of unable to do that with the current tools. Is it also possible to import or use within the engine some other types of particles other then .particle?

Or is it possible to convert some other types to .particle? And is it possible to create an particle from several images because if I would want to create a realistic smoke that would go up and would lower its intensity at the bottom, I would most likely needs something that would be best represented with .gif style images.

Yes this would be very nice indeed to have

yes, it would be great.

i think that the engine needs a more advanced system of particles. something like Particle Universe,
SPARK, etc.
such little things make a perfect engine. i think it's a matter of time. system of particles is in any game, it is a big influence on the result.

Offtopic: i am currently playing in the Torchlight and try to think of how to make similar effects in EE.

Yes well Particle Universe would be great cause i have still a license for it

you can use manual particle drawing "particle draw begin" or something like that
plus you can use VI - vertex index buffer with custom shader

I'm sorry but I don't really understand what you mean?

in "particles.h" you have functions for manually drawing each single particle

also when you own the license you can write custom shaders which you can use together with 'VI' class which also can be used for manual drawing