How to create a Japanese Nintendo account
Go to account.nintendo.com and log-off your current account is necessary. From there, register as normal with an e-mail address you have not yet registered to Nintendo. For country/region, select Japan. Submit, then verify the e-mail. Bam, you have your account. Now create a new profile on your Switch and log-in with that new account.
How to add funds
You cannot use your credit/debit card or Paypal account to add funds directly onto the account if you don't have a Japanese bank.
However, you can easily purchase prepaid eShop cards. Three commonly recommended sites are linked below.
Each ones have their pros and cons.
SeaGM, while the cheapest of the three, can deny purchases with no real pattern to explain why.
JapanCodeSupply will take additional time to process your purchase if this is your first time with them
PlayAsia is the most expensive of the three.
Once you have a code, go into the eShop on your Switch and scroll to the option below Nintendo Switch Online. From there, you can input the code.