The Laser Interferometer Space Antenna gravitational-wave observatory will be limited in its ability to detect mergers of binary black holes in the stellar-mass range. A future ground-based detector network, meanwhile, will achieve by the LISA launch date a high sensitivity to such mergers. In our latest paper in Physical Review Letters we propose a method to use the information from the ground to revisit the LISA data in search for subthreshold events. We show that the signal-to-noise threshold employed in LISA can be significantly lowered, greatly boosting the detection rate. For example, we demonstrate that while current LIGO binary black hole merger detections would have evaded detection by LISA when employing a standard signal-to-noise threshold of 8, this method will allow us to easily (possibly) detect an event similar to GW150914 (GW170814) in LISA. We estimate that the total rate of stellar-mass binary black hole mergers detected by LISA can be boosted by a factor of order 4 (or greater than 8) under conservative (optimistic) assumptions. This will enable new tests using multiband gravitational wave observations, significantly aided by the greatly increased lever arm in frequency.