You can export your Logins to a CSV file and then import them in Apple's Passwords app. Note, the Passwords.app only allows storing of Logins. Notes, Credit Cards, Software Licenses and Bank Accounts cannot be imported to Passwords.app.

Exporting to CSV

On a Mac:

1. Open and unlock Secrets.
2. Choose File -> Export… from the menu bar.
3. Choose what you want to export (All items, Vault items or selected items).
4. Choose "Comma Separated Values (.csv)" for the export format.
5. Choose a location to save the file and click "Save".

On an iOS device:

1. Open and unlock Secrets.
2. Open the app's Settings.
3. Choose Import/Export and then "Comma Separated Values (.csv)" under the Export section.

Secrets will ask for your passphrase and export your data.

Tweaking each column's name

Secrets ability to export to CSV predates Passwords.app ability to import it. As a result, some adjustments are in order.

1. Unzip the .zip file exported above.
2. Find the credentials.csv file and open it with a text editor.
3. Replace the name of each column of the CSV in the first line according to the table below:

   Secrets	Apple Passwords
   name	Title
   website	URL
   identifier	Username
   secret	Password
   notes	Notes
   oneTimePassword	OTPAuth

4. Save your changes.

You can finally import this file into Passwords.app.