def add_to_list(existing_list, inp_list):
    """ Add input list to an existing list. The existing "list" and input "list" are checked to see if they are list instances, 
        and if not configured to be a lists. The existing list is extended with the input list.

        :param existing_list: Existing list, which needs to be extended. Can be made of heterogenous objects.
        :paramtype existing_list: list

        :param inp_list: Input list. Can be made of heterogenous objects.
        :paramtype inp_list: list

        :return: Extended list.
        :rtype: list

    """
    if inp_list is not None:
        if not isinstance(inp_list, list):
            inp_list = [inp_list] # make to list

    if existing_list is not None:
        if not isinstance(existing_list, list):
            existing_list = [existing_list] # make to list
        existing_list.extend(inp_list)
    else:
        existing_list = inp_list
    
    return existing_list