I am using the telepot python library, I know that you can send a message when you have someone's UserID(Which is a number). I wanna know if it is possible to send a message to someone without having their UserID but only with their username(The one which starts with '@'), Also if there is a way to convert a username to a UserID.

You can't send message to users using their username that is in form of @username, you can just send messages to channel usernames which your bot is administrator of it. Telegram bot api uses chat_id identifier for sending messages. If you want to achieve chat_id of users, you can use telegram-cli, but it's not easy at all because that project is discontinued and you should debug it yourself. in your case you should do following command:

> resolve_username vahid_mas

and the output will be something like this:

{
  "user": {
    "username": "Vahid_Mas",
    "id": "$010000006459670b02c0c7fd66d44708",
    "last_name": "",
    "peer_type": "user",
    "print_name": "Vahid",
    "flags": 720897,
    "peer_id": 191322468,
    "first_name": "Vahid",
    "phone": "xxxxxxx"
  },
  "online": false,
  "event": "online-status",
  "state": -1,
  "when": "2017-01-22 17:43:16"
}