Refering To Sepeate Databases

Searching returned too many relationship questions so appols if the answer was staring in my face.

A = big company database (updated monthly by the big cheeses)
B = small "team" database (continually, manually updated by ground level staff)

on the first of the month A is updated

B contains various records from A, but doesn't link in any way (nothing changes when A changes)

I want to change this - I want a query whereby B looks in A, and updates:
1: the balance (how much the customer is in credit or debt)
2: the identifier (a number used to identify customer treatment - e.g 1 - customer is blind, send brail info, 2- customer always pays on time, 5% discount - this changes day in day out)

So lets say, for simplicities sake there are 3 fields in both databases, the 2 above and a number primary key.
How would I do it? bearing in mind both databases are seperate.

(note/edit)
also note I only want to run the query at the start of the month - as reflecting the info of A would result in the data in A+B being 31 days out of date at the end of the month (as opposed to just A)

(useless info :) )
The idea of this is to reinforce integrity in B at the start of the month - ensuring everything is correct, whilst still allowing workers to update the info during the month (allowing me to make sure the highest value debt is worked first)