In terahertz (THz) wideband systems, beam-squint causes deviations in the generated beam directions at different subcarriers due to the use of subcarrier-independent analog beamformers. In order to combat the performance loss due to beam-squint effect, this work employs spatial path index modulation (SPIM) to improve the spectral efficiency (SE) performance of the overall system, thereby compensating the loss due to beam-squint. Specifically, SPIM allows the transmission of additional information bits to the receiver via modulating the indices of the spatial paths. The proposed approach is evaluated in a THz integrated sensing and communications (THz-ISAC) scenario, wherein the beamformer design allows generating multiple beams toward both radar targets and the communications user. Numerical simulations demonstrate that the proposed approach exhibits significant SE performance even higher than that of the use of fully digital beamformers without SPIM in the presence of beam-squint.