Three of the seven rocky planets (e, f, and g) in TRAPPIST-1 system orbit in the habitable zone of the host star. Therefore, water can be in liquid state at their surface being essential for life. Recent studies suggest that these planets formed beyond the snow line in a water-rich region. The initial water reservoir can be lost during the planet formation due to the stellar activity of the infant low-mass star. However, a potential subsequent water delivery event, like the late heavy bombardment (LHB) in the Solar system, can replenish planetary water reservoirs. To study this water delivery process, we set up a simple model in which an additional 5 -50 M⊕ planet is embedded in a water-rich asteroid belt beyond the snow line of TRAPPIST-1. Asteroids perturbed out from the chaotic zone of the putative planet can enter into the inner system and accreted by the known planets. Our main finding is that the larger is the orbital distance of planet, the higher is the amount of water delivered to the planet by an LHB-like event.