100% possible aslong as you have a decent relevant page for each specific other area the company covers (keep the one address) and add it in the footer of your website with a map, your opening hours and contact details, google loves consistancy so make sure this is the same across all your networks, now you can extend the google map locations to the specific area with the one address so create linking strategies for your area specific pages and include these in the footer also.
Make sure content is unique copy and pasting from other local area pages will not cut it anymore, little tip try include some local FAQ's regarding the area make it unique to each area and you should be good.
A doorway is simply a landing page, your not going to do this, your going to give relavant information but still show your call to actions above the fold
hope this helps. I can expand on this more but think this should be enough.