BeOS doesn’t include a free-standing icon file format — all BeOS icons live inside file attributes, so you can’t have an icon that’s not attached to another file. If you want to distribute icon packs with other BeOS users, just make a bunch of zero-byte files, such as empty text documents or empty folders. Give each of them one of your icons. Put them all in a folder, and zip up the folder. Since zip preserves attributes, it preserves your icons. Other users can simply open up the filetype dialog for each iconized file and drag or paste it into another filetype dialog of their choosing.
For an example of this technique in action, check out the Jean-Paul Sartre BeOS Icon Action Pack.
If you want to create a bunch of blank icon files at once, you can just tap Alt+N in the Tracker a bunch of times, or open a Terminal and type something like:

touch icon1 icon2 icon3

However, the emerging standard seems to be that icons have a filetype of application/x-vnd.Icon, so it may be easiest to just create one blank file with this type, and then tap Alt+D to create duplicates of the same type.