Yes. A Pokémon GO Plus + and a smartwatch can both be paired, and will both stay paired to Pokémon Sleep at the same time.
If both are paired, you can choose which sleep data to import into Pokémon Sleep.
However, if the sleep data overlaps in time, you will only be able to import data from one of the sources.
When importing sleep data from a smartwatch, you will need to select a data source (such as Apple Watch's built-in sleep tracking, the Fitbit app, Samsung Health app, etc.).
Only sleep data from the selected data source will be imported.