I guess its possible but not with the theme creator.. it cannot edit icons.. sony ericsson says icons are implemented in the software of the phone. You cannot see the icons files when its connected to the PC either. Some files are mysteriously hidden in "My Computer" because the filesystem of the phone is not FAT or NTFS.
Only Mobile Operators like Vodafone etc has themes with different icons which means they probably have software for it that is not available for the public.
To edit icons an advanced program is needed or something that can "hack" the icons.
I'm afraid that icons are implemented into the firmware of the phone. I aint gonna mess with that
Now about the theme creator,
You can convert the K850i theme to any other Sony Ericsson model in the program (theme creator). It will give a message its not compatible but it will try to convert it. After that you have to check all the settings yourself to see if its still all in place with the pictures, colors etc. Because converting is not 100% perfect. Mainly because every phone model is different, each phone model has different screens, buttons, features etc.
you are lucky because the nokia editor can litteraly edit EVERYTHING and i mean EVERYTHING.. from icons, sliders, buttons, colors, pictures, etc etc etc. So probably it will take me much longer time to edit your theme, but the result will be a true amiga phone i can almost assure you that.
Update : Found another 3rd party Sony Ericsson Theme Editor
<-- Like i thought, icon editing means hacking phone, modding. No way i'm gonna do that
You really don't wanna destroy your phone
my k850i is just brand new so its a no no