Multiple localization in one DB

Hi, I'm in charge of marketing and not a technical person.
Although I read some of similar topics, please allow me to ask about multiple localization in one DB.

Now we have a customer using NAV in several Asian countries (Japan, Taiwan and China) with one DB.
The above countries do not have MS official localization so they are using W1.

Now their US office would like to start to use NAV accessing the same DB.
US has NAV official localization so we would like to merge US localized objects to current W1 DB. We don't intend to merge all localized objects. (It will take huge amount of cost and time... ) What we only need to merge is Sales Tax related objects.

Below are my questions;

1. How can we merge the objects?
Do we need to change the localized objects to 50000-99999 ID objects?
2. How many days do you estimate on average to merge the objects? (Only sales tax related objects.)

Thanks.

Miyuki