Howdy, all.
For some of you this may be old hat but I've been trying to figure it out/find it for years. If you already knew this, I apologize for taking up your time.
For those who are still looking, here's how to make the + and - signs show in spreadsheet programs (Excel, Google Sheets, Free Office, etc.) for your character sheets and game notes:
In the spreadsheet, select the cells you want to display numbers with these signs. Click on Number on the top bar and select Custom from the drop-down:
Choose the General format at the top of the Format window and paste this over the word General:
+#,##0_);-#,##0
Save the spreadsheet. AFAIK, once you save a document with this formatting in it, it will be available in subsequent sheets you create.