It is my understanding that a single dose of the Janssen vaccine is still recognized until 2021-01-02 [0]. After that, you need to have an additional dose of any EMA-approved vaccine.
I assume you would be able to get a booster off-label in the Netherlands.
[0] https://gruenerpass.gv.at/geimpft/ "03. Jänner 2022 ist das Impfzertifikat einer einmaligen Janssen Impfung nicht mehr gültig."