Well there is not a lot to say, since I don't know what you want to do.
But the approach would be to use a program to manipulate atomic structures, such as Vesta, or use a code, e.g. python, to construct the particle you are interested in. Then you save the atomic coordinates and the atom types in a xyz-file. The xyz file type is standard in most crystal visualization programs.
The you can use the loadstructure function with the periodic keyword set to false to load in the xyz file as the atomic structure used for the refinement.