Its possible add a Tooltip in MapMarkerGenericRadius?
If yes, how?

As I remember, no. You can make marker as vending marker and choose a vending machine name - it would be your tooltip

Thank you, does the vending mark need to be updated every time a player connects?

If you change text/.. you.need to send network update. If player just connects and there is an existing marker, then no

Is there any way to change the color / size as MapMarkerGenericRadius?

Yes:

MapMarkerGenericRadius marker = GameManager.server.CreateEntity("assets/prefabs/tools/map/genericradiusmarker.prefab", position) as MapMarkerGenericRadius;
marker.alpha = 0.3f;
Color col1;
ColorUtility.TryParseHtmlString("#278427", out col1);
marker.color1 = col1;

marker.color2 = Color.black;
marker.radius = 100f;

marker.Spawn();
marker.SendUpdate();