Different Fields in Sections of the Same Volume

Dear experts,

I’d like to gather some information about different ways in which fields can be set in Geant4. As it is a complicated issue, I’ll try to explain it as clearly as possible.

In a direct approach, given that a single volume exists, fields can be assigned to it using the FieldManager to put, for example, a G4UniformMagField.
Another way is to use a field map, as demonstrated in the example purging_magnet. This is then based on the coordinates and the particle positions.

But what happens if one would need to set up different fields in sections of the same volume. I’m referring to a case where an external geometry is imported to Geant4, for example a beamline of an accelerator. Along this beamline, different magnetic fields would guide the particles. But let’s assume the external geometry contains no information on single elements (such as dipoles or quadrupoles) - would I need to set up single volumes along the beam, wherever it would pass a magnet in which to establish the fields? I’m wondering if such an approach would actually be reasonable or even possible, as all these volumes would have to be located inside the already existing geometry (although that’s not impossible, I guess).

Every thought on this problem is most welcome - thanks in advance!