The conventional quasiclassical approach to magnetotransport in magnetic multilayers and magnetic granular solids is found to be suitable only for collinear magnetization configurations. A quantum treatment of electron spin is proposed to improve the quasiclassical theory to be applicable to arbitrary magnetization alignments, in which the electron distribution functions, as well as the electric fields and currents, need to be regarded as spinor matrices which are off-diagonal in spin space of conduction electrons. An extended Boltzmann equation has been used to derive the two-point spinor conductivity in magnetic inhomogeneous systems. The result obtained is found to be identical to that obtained from the real-space Kubo formula, indicating a close link between the reformed quasiclassical approach and the quantum theory. Angular dependence of the current-in-plane magnetoresistance in magnetic superlattices is discussed.