This question might need to be moved to the excel section.
I also do not have answer to your question but can you try something.
Can you record a macro in excel. While recording can you protct the sheet and then unprotect a sheet.
Then look at the resulting code.
Then you can use this in revers
Unprotect sheet
add to you cell
then protect again.
This is how I learnt to do it in WORD.