No, u cannot defrag your phone memory, cause there is no need for it.
Btw, AFAIK, only hard drives can be defragmented, not flash memories. Defragmentation in hard drives causes the different parts of a file to come together. This makes it easier(and faster) for the drive to access the file. There is no such need in flash storage since there are no moving parts. I'm not sure about all this though.