Bakup backrests do stand alone backrests . You need to call them to insure it will be compatible with the aftermarket seat. But, it should be as long as your bike isn't a factory "lowered" model with the seat being one piece front and rear. http://www.bakupusa.com/bmw_models.html
Scroll to see the correct rest for your year bike. Not inexpensive.
If I may, you just got the bike and you came from a whole different type of bike. (Seating posistion) give the bike a couple thousand miles before you start spending money as you are now using a whole different muscle group while riding. (unless you have chronic back issues)
The same would be true for an upright bike rider switching over to a cruiser bike with forward controls, laid back seating.
While at Ozzie's have them go over the operation of all the bells and whistles the bike has so you know WHAT the bike has and how it works. The PO may not be the best at explaining.
The BMW's do much better with higher than normal tire pressure as well. AND coming from a cruiser, you may be used to shifting at a lower rpm. Try upshifting above 3500 or so and know that 5th and 6th are very tall gears. 6th being a true overdrive, so going over mountain passes, the bikes are happier in lower gears .
The bikes like to rev.
Let's say you are going over Pacheco Pass. The bike will be ok to do it in 5th, but in 3rd, it will be much happier. You'll have the power on hand to pass safely and really enjoy it.