Can I start such a contact with one researcher that does not know me in one online conference?
Conferences, including online ones, are for liaising with people and talking to them. There is nothing wrong with what you suggest. There is always the chance the professor might ignore you, not be interested or not know, but that can happen anyway.
I do not know what functionalities the conference offers, but a brief text or email asking if the professor could spare the time for a chat on X and Y seems like a good way to start. Try not to ask during the questions after a paper presentation or, generally, the proceedings of the conference, but a workshop might be an alternative. Personally, I would just text or email him.