There is not easy way to apply antialiasing to selected objects. Usually antialiasing is applied to the whole backbuffer or renderbuffer.
You might want to try an approach based on post-processing where you use a FXAA pass only for a certain group of objects. So you render these objects first, apply FXAA and then render other objects. This approach is not perfect since you might have problems with missing values in your depth buffer. More information about that topic right here: