Can add-in get context for host web's site collection root web?

You can as long as your app has been granted the required site collection scoped permissions. 

Here's some demo code I wrote that shows how to build your context

var rootWeb;

    function getSiteCollectionTitle() {
        var scServerRelativeUrl = _spPageContextInfo.siteServerRelativeUrl;
        var clientContext = new SP.ClientContext(scServerRelativeUrl);
        rootWeb = clientContext.get_web();

        clientContext.load(rootWeb);
        clientContext.executeQueryAsync(onSuccess, onFail);
    }
    function onSuccess() {
        alert('Title of Root Web:' + rootWeb.get_title());
    }
    function onFail(sender, args) {
        alert('Error: ' + args.get_message());
    }