Can you cream my back? Don't say this, not only is not right, it also sounds kind of dirty/sleazy. I'll let you use your imagination as to why that might be the case. 'Smear' also isn't normally used in this context. 'Can you rub some cream on my back?' is how to say it, or 'Can you put some cream on my back?'

I think that the most common way, said in the relevant context, is simply "Can you get my back, please?"