Providing tabulated data to a SQL Update

Newbie question.
Task: Transform some SQL records where Base64toHex conversions are needed on some Active directory entries which need to be read.

So is it possible to have C# code / function called in the SQL form query? e.g. in https://www.linqpad.net/#db

or is this best done as a two step process?, i.e.
C# code to add some temporary SQL Table (old value, new value) and then use a Linqpad SQL query to iterate through the temp table making the updates ? Could a CSV table be used ?

Advice sought, thanks