Insert a void space in gravity forward model

I want to insert a void space ~300 m below the topography surface. The xyz of coordinates of the void space is given in a text file and it is in much smaller discretization than the mesh. I was wondering would there be anyway (something similar to active_from_xyz or surface2ind_topo) to obtain the active cells using the SimPEG functions? Otherwise, could you please point out how to find the indices of specific cells that corresponding to the void space given in the text file?