To import a coordinate dependent permittivity in HFSS, you can the edit the material properties accordingly:
(For some reasons currently not able to upload an image. You can try to follow the workflow below)
Go to View/Edit material >> Enable Spatial modifier >> a column appears in the properties of the material. Suppose, if you want to give coordinate dependency in the relative permittivity; beneath the spatial modifier column a drop down box corresponding to the permittivity appears- Edit >> Edit Spatial Modifier pop-ups >> enable 'Use spatial dependent dataset' >> in the drop-down box : Add/Import dataset..>> another window pop-ups for importing/creating data sets.
Hope this helps.