Maybe write a program to do it. Or maybe not, unless you expect to be repeating this task many times.
The "best" way would be one that you can actually do. If you don't know how to write a program or script that runs on the Mac and tells Excel to export CSV then parses the CSV into plist form, then that's not really an option. So you should describe what you can do, and post that.
We have no idea what your experience is, what your other skills might be, or how many times you have to perform this conversion to plist. You could start by explaining what the data is, how it got into an Excel worksheet, and why you have to extract it from Excel. If the data originates somewhere else, you might be better off going to the original source, rather than trying to suck it all through the Excel sodastraw.