Is customer Bank Account/ACH information available to the API in anyway? use case my customer has a list of thousands of customer Banking info in their own DB and want to update the NS customer record with that info on a regular basis. customer can update that info through a portal they created.

That info is stored in custom records if you're using the Electronic Payments bundle. Those records should be scriptable. They start w/ "custrecord_2663". You may be looking for the Entity Bank Detail record.

This gives me the answer I am looking for.