Playing with steam in offline mode

Hello, I will be straight with you, I would like to purchase a steam account to play on BeamMP, but the issue is that all these accounts are offline (steam in offline mode), which means that I will be able to update the game and it will be a certified copy, however, I will not be able to play online. But BeamMP is not technically an official online mode for the game, so maybe it is possible to play? Did someone try this? Thank you for the answers in advance.