Entering the data manully into a goole sheet will break the integration?

I need to add a row of information to the excel spreadsheet linked to my jotform. If I add a line of text, will it impact future data that automatically populates the spreadsheet?

You can achieve this using the Spreadsheet to Form widget.

User guide : How to Use the Spreadsheet to Form Widget

You can use your Excel sheet as the data source to linked the data to the form. 

But please take note that you will need to upload the Excel sheet first. So if you have update on the Excel sheet, you will need to upload it again to the form so that the updated Excel sheet data will be use.

I hope this help. Let us know if you need further assistance.

On the other hand, it could be possible also you were looking instead to downloading your form data into Excel sheet. This is possible as well.

User guide : How to Export Form Data to Excel

Hi.
This isn’t quite my question. We use jotform to register people for a
course. It automatically populates my excel spreadsheet. On occasion I have
someone who registers by snail mail and I have to manually enter the
information into the excel sheet. If I enter the information manually on a
row will it mess up future registrations.
Cathy
...

I believe you are talking about the Google Sheet integration. Normally, we do not recommend our users to modify their integrated google spreadsheet because it could possibly break the integration resulting in spreadsheet stop syncing the new submissions. 

If you face any issues with new submissions, I can only suggest you re-integrate your form with Google Sheet to fix the issue.