[Wien] Supercell setup

[Shu Miao]

Dear Wien users,
I have a very basic question concerning the supercell setup.
My material is #166(R-3m) with O at (0,0,0.26), Mn at (0,0,0) and Li at
(0,0,0.5). [These are hexagonal coordinate, and they are easily converted
to (0.26,0.26,0.26), (0,0,0) and (0.5,0.5,0.5) in rhombohedral system.]
I want to replace half Mn with Ni. So seems I need to build a supercell.
I created the case.struct by StructGen. Then run "supercell" from the
command line. Only specified the dimensions to double along x,y,z and kept
other default setting. The program wouldn't run, here is the displayed
message:
>>>>
 Program generates supercell from a WIEN struct file.

 Filename of struct file:

 Fatal Error occured:
 Unknown lattice type: R

 Program terminated.

0.001u 0.000s 0:00.00 0.0%	0+0k 0+0io 61pf+0w
>>>>
I thought this was because all atoms were on their ideal positions as told
in the FAQ. I tried to modify the structure from StructGen. Putting lables
on Li/Mn, it doesn't work (it should not work since only one position
for Li/Mn in the cell). Displacing one of the O position slightly, this
changed the O multiplicity from 2 to 6. But when run supercell to double
the size, above message appeared again.
I know there must be something wrong in my method to break the symmetry.
But I don't know where this mistake was.
So the question become quite basic--how to make some atomes different in
the case. struct file or in StructGen? I worried that the multiplicity
will also work on the modifications so that more positions thant I expect
will be changed, even if I can make some change on the file. Who can give
me advice to have the "supercell" run?

Thanks a lot