Its totally fine to have your door open if you want it open. As a bonus, anyone that just wants to chat with a random person will come to you because you're the only one with an open door
one suggestion for keeping a conversation going is called Nouning. Ask an open ended question (How have things been these past few weeks for you?) And almost always the person you're talking to will mention a noun- a person, place, thing, event, activity, etc. When they finish talking, either talk about your own experience with that noun or ask another question that has something to do with the noun. Some nouns are more important than others so keep track of what you think are the most interesting/important and ask about them when conversation dies down. This is the process by which most people carry on a conversation, and after a while it should get easier to remember the nouns, and later even internalize the whole process, so it's an subconscious sort of thing.