As far as I’m aware, the API only gives developers the ability to create feed items, not transaction entries.
I’ve just come across this quote from Simon though, about the open banking standard & what it’ll mean for Monzo users in the future
in the meantime, I expect a manual import would be complex (mapping the columns in your csv export to Monzo’s database) & too time consuming to do on an individual basis. That’s assuming that your file even has all of the information that Monzo needs for transaction entries but hey, there’s no harm in asking 