I found nice feature in google maps for android - save maps for offline use. But looks it saves maps only to internal memory. How to save maps to SD card?
It is definitely possible to do what you want, although it needs a couple of things done, which may or may not be viable to you.
You will need to use Link2SD to link all data related to Google Maps onto your SD card (or just the data(ext) in your case should do the job).
However, using Link2SD requires (1) your phone to be rooted, and (2) a second partition on your SD card, which will be used to store "linked" applications.
Complete details can be found within this answer: