Thank you all for such quick response - i appreciate it!
OK, probably i am going to try it with hacking some HG casette instead of doing something on the old hub and its freewheel body.. (maybe some music will help yes..)

This will be probably my first victim: http://www.kaktusbike.sk/hg-30-altus-11-28-7-kolo-3250
There is a minor? prob. with this casette for this purpose. My old lockring is 13T .. But the goal is to get that 28T. Than the overall gage..
Than the distribution (distance) and count of the tooths on cogs (for the splines) How many splines will hold the cogs than.. :-/ (so it looks like i have to do some deeper research...)

Asi was asking for a type of casette.. (i dont really know, there is number of T and "index" with arrow written on each cog..)
And the treads on freewheel body are on the splines..
here are pict of it if that helps..
http://imageshack.us/photo/my-images/507/pict2877x.jpg/
http://imageshack.us/photo/my-images/15/pict2875x.jpg/

Thank you once more for your reactions, its very helpfull!