Saving individual sheets as .csv isn't saving plus symbols at the start of cells..

That's not technically correct. If you open the csv file with notepad, you'll see that the plus-signs are there.
The problem arises when you open the file with Excel in the normal way. Excel interprets the plus-signed numbers as numeric...and discards the plus-sign.

Try this:
Data.From_text...Select your csv file
...Check: Delimited.....click: Next
...Check: comma........click: Next
...Select the last column....check: Text
...Click: Finish

When imported as text, the final column retains the leading plus-sign