Wait, wait, hold on... yes, I know Zelda II didn't come out on a cart in Japan, but it did come out on disk.
By 'translating' the game into Japanese, I presume you mean taking all of the Japanese text from the FDS version and putting it into the US cartridge version.
Don't get me wrong, I've beaten both versions, and the different levelling systems do make it a different experience. Not a very big difference, mind, especially when you get near the end, but different.
Looking at your history, you seem to have done work on other NES games before, yet now you need help changing the alphabet to kana? Also, you don't want to add it to the PPU Viewer in FCEUX, you want to put it in the ROM, though judging by what I see in the PPU Viewer it may be a tight fit.
Get Tile Molester and try pasting in the graphics you want, then go from there. Forgive me if I sound a bit strange but I do wonder how many Japanese natives are reading RHDN in the hopes that someone will turn the US version into Japanese...
It's not that I think it's a bad idea - hell, the US version IS the better version - I just don't think you'll get much of a response for this. But hey, it's up to you.
Oh, and couldn't you copy the katakana from the Zelda II FDS disk instead of Zelda 1? Don't think it matters, but whatever. If I was doing what you're doing, the first thing I'd do is get into the FDS version and see what's what.