Just create the new field in your laptop version of MC giving it the same flags in the field configuration window, and making sure that it is the same kind of field (numeric, list, semi delimited etc.)
When you synch the files to the laptop as a portable device (I assume using a smartlist to tell it which files to send), make sure that in the handheld options you have "delete files not in list" ticked, and then any files not in that smartlist will be deleted from the laptop next time you synch. You dont need to worry about which fields will synch or not; the whole file is copied accross, so any fields in it will be visible in the laptop library (as long as you have created those fields in that library as outline just now).
I do a similar thing to you. I make sure that "get playstats from handheld on connect" is ticked so that my main library gets the "number of plays" info updated from my portable hard drive upon synch.
If you make any other changes to the files on your laptop (tagging stuff), I dont think there's any way of having that info being synched back to your main library upon synch. You would physically have to copy the laptop files over the top of the ones in your main library, and then in your main library "Update files from tags" to get the library to see the info you tagged on your laptop. But that is hassle and messy, so I dont do it. I just treat my portable library as a convenience at the moment, and resist doing any tagging at all.