As I indicated, some of the answer to these questions probably should be context dependent. Where are these DEC numbers coming from? How are they being used?

Am I better off just entering each into the calculator in PROGRAMMER mode and translating each individual number to HEX? Or is it possible to translate the whole list via excel?
the "best" approach might be to abandon Excel in favor of a programming language with good support for very large numbers. But, I get the impression that you are unfamiliar -- not only with Excel -- but other programming languages as well.

If it must be done in Excel, we probably need to think through how to process a very large "number stored as text" (large enough that we can never allow it to be stored as a number) and convert to a hexidecimal string. I will think on it, and see what I can think up.