can user define a shape of particle

Yes and No.
Bc of the contact detection the only fast method is to use spheres (contact=x1-x2

The contact detecton is than performed for the particle-particle contact.
Watch this video for an eye candy:
http://ubuntuone.com/38FXJ099ZkhNliee5si1DP

Greeting,
Chris

*not in public release, yet

in theorie you could implement contact detection for superquatrics, but until now nobody have had the time and the founding to do this.

Bc of the contact detection the only fast method is to use spheres (contact=x1-x2<=radsum?).
One possibility is to "glue" many spheres together using fix rigid or fix multisphere*.
A simple box could look like this:

or more complex shaped a stone:

greetings,
Christian.

*not in public release, yet

is there any example/ or guides onhow to use ellipsoid particle?