There is an Orion request that xtgeo should resample a seismic cube into a 3D grid property. There are differerent ways of doing this, but starting with a "nearest node" should be a simple approach. Other approaches may be cubic or linear.

Possible code outline:

grd = xtgeo.grid_from...(...)
cube = xtgeo.cube_from_...(...)

grid_property = grd.sample_from_cube(cube, interpolation="nearest")

EDIT:

grid_property = xtgeo.gridproperty_from_cube(grd, cube, ...)