You can easily export your manga library from Kenmei as a CSV file and then import it into MangaTime.
Here’s how to do it step-by-step:
Open kenmei.co and log in to your account
Tap your profile icon in the top-right corner of the screen
Select Settings from the dropdown menu

Once you’re on settings page, tap on the Dashboard tab

Scroll down to the List Export section, tap on Export and when it finishes on Download

The CSV file containing your manga list will be downloaded automatically
Go back to MangaTime
Open the Profile tab using the bottom navigation bar
Open up the Settings pageby tapping on the gear icon in the top right corner
Press on Import from Kenmei
Tap on Select CSV File and pick the downloaded file from your downloads
Then press on Start Import, wait a few seconds (based on how big your library is) and TADAAA 🎉