Unfortunately, this is simply not possible.
Samsung does not offer a sirius/xm app for their smartub enabled tvs. This is a limitation of the tv firmware.
These tvs are not like your computer or ipad, and do not have a full scale "app store" where items can be added at will. There is a small selection of apps available, which are the only options that can be used on the tv. The only way that would ever change is if samsung designs and writes a specially adapted version of the sirius/xm app to operate on the firmware of your particular tv. If/when that happens, it would be included with a future firmware update.
At this point though, I would not expect that to ever be release, as they very rarely ever add new functionality/apps like this.
Your best option would be to utilize an external streaming device such as a chromecast, roku, appletv, fire tv, and get all the same functions your tv already has and many more, including the sirius app.
Best regards, ***** ***** for the news,