Hi Daniel,
Unfortunately, our tools don't have a good way of discovering keywords your site is ranking for in that way. Our tools are set up to help you gauge your site's performance for keywords which you enter, rather than finding keywords which are already doing well and presenting them. It's a request we hear a lot, and it's definitely something the product team is looking into, but for now we don't have that feature in our toolset, I'm afraid.
In the meantime you can find keywords you're getting clicks and impressions for in your Google Search Console. You can download the list from there and upload those to your campaign.
I an also recommend using Keyword Explorer to find more keywords with potential to add to your campaigns.
Let me know if there's anything else I can assist with!