Is there a way I can paste a report into a pre-set Excel work sheet ?

Because many widgets do not support translation, I have had to had insert some fields 2x, once for each language, then use conditions so that the French field shows when the user chooses French, and the English when he chooses English. I would like to find a way to combine the answers to each in a single field in my reports, and one method would be to add a 3rd field in Excel which combines the 2. To do this I would need to be able to predefine my Excel sheet. Can I do this ? If not do you have any other ideas ?

Once you create Excel report you can create new column and add data from French and English widget fields to new column. However you will need to do this manually.

Other possible way is to add new field to your form and pass data from both French and English widget fields to that field. This way you will have all data inside of that field when you create your excel report and you can remove original French and English widget fields from that report, so that there is not duplicated data.

You can remove fields from reports by un-checking them on Submission page of the form. 

Inside of this guide you can find how to pass field value to another field with conditional logic:

https://www.jotform.com/help/286-How-to-Pass-Field-Value-to-Another-Field 

Hope this will help. Let us know if you need further assistance.