Is it possible to allow backoffice service to connect databases without any APIs?

Connecting to database doesn’t need APIs (I mean REST APIs) , REST APIs act as an interface between client and server. You may have libraries/utilities/connectors to connect to database from your Microservice (which has the APIs exposed)

Is it wrong? or breaks the concept of microservices?

To still answer the question, microservices may share the database and you may pull some data from a different database if your microservice does not have it’s own. It certainly won’t adhere to the principles of microservices architecture but lets consider them guidelines and recommendations (which you can still violate) !

This article may help you decide better

https://microservices.io/patterns/microservices.html