write a report to copy all customers from one company to another company.

If you want to copy just the customers (no related data, such as sales/purchase documents, ledger entries, comments, prices, etc.), then it's quite simple (personally, I'd use a codeunit or a report with no data items and code in OnPostReport):

CustomerCompA.CHANGECOMPANY('SOURCE COMPANY NAME');  //can be skipped if executing from source company already
CustomerCompB.CHANGECOMPANY('TARGET COMPANY NAME');
IF CustomerCompA.FINDSET THEN BEGIN
  REPEAT
    CustomerCompB.COPY(CustomerCompA);
    IF NOT CustomerCompB.INSERT(FALSE) THEN BEGIN  //could be that a customer with the same No. already exists
      TempCustomer.COPY(CustomerCompA);  //if that happens we want to show
      TempCustomer.INSERT(FALSE);  //a list of customers that could not be copied
    END;
  UNTIL CustomerCompA.NEXT = 0;
END;
PAGE.RUN(0, TempCustomer);  //display the customers that could not be copied

Variables here:

CustomerCompA - Record - Customer
CustomerCompB - Record - Customer
TempCustomer - Record - Customer (Temporary=Yes)

If you want to copy related records, then you need to copy those related records in the same way.