Hi Sarah -
It's actually possible to add text in the middle of existing descriptions, by using the "Find & Replace" function. Here are the steps to do so...
1. Select "Find & Replace"
2. In the bulk edit "Find" text field enter a few words of text (from the existing description) that stands just before where you'd like to add new text. (Note: The text will be highlighted as it is typed).
3. In the bulk edit "Replace" text field enter this exact text (if you'd like to keep it) along with whatever new text you'd like to add after. (Note: The text will appear green as it is typed).
4. Click "Apply"
5. Click "Sync Updates"
Hopefully, this makes things clear, but if you have any questions please let me know.
Thanks!