Definitely, and we strongly encourage students to do so. The APA requires Clinical PhD students to take one course each in the cognitive-affective bases of behavior, the biological bases of behavior, and the social bases of behavior. In addition, it is possible to expand one's research experience by working with a faculty member in another area.
Clinical PhD students may concurrently complete the requirements for a PhD in Neuroscience.  Detailed information about the program can be found at Neuroscience Program.
Clinical PhD students may also complete the requirements for a Certificate in Behavioral Genetics.