Networks

Hey, first time poster, been hanging around for a while.
Been in tech support for about eight years, am now in more of a network admin/programming role with a growing position.

we deal in the insurance business. we have a corporate office and about fifty satellite offices. all pc's here at corporate and satellite offices are on our network. We have a fax server, mail server (but our mail client is thunderbird) domain server, and document locator server. We have managers that have laptops that travel from location to location.
The issue ( or ease for their sake) is when they are on wireless, we give them the outside IP to essentially be on our network, however, when they come to the corporate office or satellite office we have to manually change the outside IP to the inside IP so they can get all access. We just started an intranet, and from their laptops they cannot get onto that and we need them too and I've been given the task to look about doing so.
So I guess it's kind of two questions, is there a way with a laptop to be on our network without manually changing IP's ? maybe like a mirrored profile or something?
And how can a managers laptop get on to our intranet? or is it possible without opening up the intranet to really everyone in a sense like internet?