At the moment the regions must match between population (as defined in the density object) and the design.
This could be modified so that only part of a population is surveyed or a design extends beyond the range of the population.
I believe, these scenarios can currently be achieved using multiple strata and allocating no abundance or no effort to the relevant stratum/strata - not tested but expect this to be a very rarely used scenario. Please get in touch if this is something you are interested in and is not working for you.