I'd like to enter my bodyfat% values alongside my weightIs this not possible in the Garmin app?
You can import that data in Garmin Connect Web using a CSV file. See more details for example here: Import Data from FitIndex Scale - Garmin Connect Web - Mobile Apps & Web - Garmin Forums
That's not what I want. I want to enter it manually alongside with my weight and see statistics on it.
Yes, that’s exactly it. Enter the data into the csv file, import it, and you will have the stats. The only other possibility is buying the index scale and have it entered automatically.