There are blank spaces at the end of the Weapons3.png file (located in /img/system/ folder), which you can paste your own sprites onto. Paste your katana sprite on there, make sure it's lined up correctly, and then make a new weapon type for it in the database, and set it to use that sprite from the weapons3 sheet.
The advantage of the yanfly plugin is that you can use as many weaponsheets as you want, and that weapons can use a specific sprite using a notetag on the weapon item. But if you want all katanas to be their own weapon type, you can easily make do with what is available by default. If not, you can chip in a couple of dollars for the plugin. If this effect isn't worth a couple dollars to you, why even have it in the project at all, right?