Question about moving money over from my US savings account for NISA contributions

I am an American citizen living in Japan long term with PR, but I have a fair amount of money in my American account from back when I lived there. Now that it seems Americans can finally invest, I'd like to start using some of my US savings for the annual contribution on that.

As it seems Americans can only contribute to the "seicho" portion of the NISA at this time, and given the annual contribution limit, it doesn't seem like very much money at all. Can I transfer this over once a year and put it into the NISA? Are there tax or other considerations? Will it even be noticed at that level?

Apologies as I see several similar questions in the past, but not exactly covering this. Feel free to point me to an earlier answer if I am bothering people with a question that has already been answered.

by Shrimp_my_Ride