Full-wave analysis of large structures that include complex and multiscale objects requires a large computer cost as, for time-domain volumic methods such as Transmission-Line Matrix (TLM) for instance, mesh size should be at least of the order l/10 and can be much smaller when details require some mesh refinement. Hybrid methods allow the partition of the structure between a complex geometry that can be analysed with a TLM meshed domain and large metal objects analyzed with approaches such as physical optics (PO) that allows a separate treatment and coarser meshing. In all cases, the TLM computational domain is limited and field computation that is needed outside for coupling with PO is required. A Kirchhoff's integral formulation in time domain is used and adapted to the TLM algorithm. It is found that the minimum spatial sampling required by the TLM exhausts the computer cost. To accelerate the procedure, some alternate sampling is proposed and comparison with the classical way is presented.