You absolutely can!
Assuming you've already installed and enabled Fetchify Address Auto-Complete in your checkout, then in your Shopify Admin
Open the Fetchify Data Validation app
Navigate to Extensions > Address Auto-Complete
Click ‘Enable UK Enhanced Address Search’
This will immediately include Royal Mail PAF Multiple Residence search results for your UK address lookups. You can disable this add-on in the same way.
When activated, you will see UK Enhanced Address Search app usage charges on your monthly bill instead of the standard UK Address Search charge. Depending on your plan, this may be a higher charge than standard UK Address Search.