Thanks for reaching out to the Community about this, byomtov.
Currently, there isn't a feature for adding percentages to existing purchase orders. I can see how this would be useful, and have submitted feedback so that it's considered in future updates.
As a workaround, currency adjustments can be made by editing a record's corresponding bill. However, before making such an adjustment, I recommend speaking with your accounting professional to verify accuracy. If you need an accountant, there's an awesome tool on our website called Find an Accountant.
Here's how it's used.
1. Go to https://quickbooks.intuit.com/find-an-accountant/.
2. In the Find an expert in section, choose what you're looking for, then use your search field to enter a City or ZIP code.
3. Select Search.
4. Review the results and find one that works best for your business. You can click on each ProAdvisor's profile to learn more information about them.

Once you've found an accountant, they can be contacted through their Send a message form:
1. Use your available text box to introduce yourself. Be sure to include details about the services you're looking for.
2. Enter the appropriate info in the Your name, Your email, and Your phone number (optional) fields.
3. Hit Send message.

I've also included a few helpful resources about working with purchase orders:
I'll be here to assist if there's any additional questions. Have a great day!