Quaternions, being vectors themselves, can be rotated. Simply extract the (x, y, z) components of the quaternion, normalize, and then rotate that vector. Construct a new quaternion from that new unit vector with the desired angle of rotation.
Multiplying quaternions together represents a series of rotations that happen subsequently. For example, a rotation of 45º about the X-axis, followed by a rotation of 45º about the Y-axis will be composed by the following:
FQuat( FVector( 1, 0, 0 ), PI/4.f ) * FQuat( FVector( 0, 1, 0 ), PI/4.f );
This would give you a result that would look similar to this:
