JSV supports only tetrahedra and octahedra. I hope to improve that in future releases.
The presence of a central atom is NOT required !

Step 1)
Display the cell with NO polyhedra specified in the input file and switch to <mode><numbers> (left image). You may also create bonds first, to make the relations more clear (right image).

Step 2)
Select the numbers of atoms forming ONE tetrahedron as its vertices (eg.: 0 3 4 7)

Step 3)
Edit the input file by adding a value for [npoly] and in the line after [poly] add the number of vertices and the atoms you selected (eg.: 4 0 3 4 7) Below an example file is given and the changed lines are highlightened.


asymmetric_unit_cell         
[cell]   5.46 5.46 10.725 90 90 90
[natom]  4          
[nbond]  3                  
[npoly]  1                     
[space_group] 121 1        
[title] Cu2-Fe-Sn-S

[atoms]
Fe    26    0.0    0.0    0.0
Sn    50    0.0    0.0    0.5
Cu    29    0.0    0.5    0.25
S     16    0.245  0.245  0.132

[bonds]
4 1
4 2
4 3                        
[poly]
4 0 3 4 7

Step 4)
Select [file][reload] or [Cells][1x1x1] to make JSV calculate the polyhedra. Central atoms are not displayed, however their labels/numbers are .

JSV ...how to create tetrahedra