As a key pillar technology for the future 6G networks, Terahertz (THz) communications can provide high-capacity transmissions, but suffers from severe propagation loss and line-of-sight (LoS) blockage that limits the network coverage. Narrow beams are required to compensate for the loss, but they in turn bring in beam misalignment challenge and degrade the THz network coverage. The high sensing resolution of THz signals enables integrated sensing and communications (ISAC) technology to assist the LoS blockage and user mobility-induced beam misalignment, enhancing THz network coverage. Based on the 5G beam management, we propose a joint synchronization signal block (SSB) and reference signal (RS)-based sensing (JSRS) scheme to assist beam alignment. JSRS enables a predict-and-prevent procedure that provides early interventions for timely beam switches. To maximize performance of JSRS, we provide an optimal sensing signal insertion and time-to-frequency allocation to improve the joint range and velocity resolutions. We derive the coverage probability of the JSRS-enabled network to evaluate its abilities in beam misalignment reduction and coverage enhancement. The expression also instructs the network density deployment and beamwidth selection. Numerical results show that the JSRS scheme is effective and highly compatible with the 5G air interface. Averaged in the tested urban use cases, JSRS achieves near-ideal performance and reduces around 80% of beam misalignment, and enhances the coverage probability by about 75%, compared to the network with 5G-required positioning ability.