Assuming you have no duplicate item numbers between the two companies and simply want to append the item master from company A to company B, then the Data Exchange can do it with a couple of tricks.
 
Assuming the Destination company is the DBA Default company (data files *.B) and the dormant company you want to append to it is designated as data files BXX, do the following:
 
Copy \DBAMFG\XX\BKICMSTR.BXX to \DBAMFG\DEFAULT\BKICEMTR.B  
Copy \DBAMFG\XX\MTICMSTR.BXX to \DBAMFG\DEFAULT\MTICEMTR.B
Copy \DBAMFG\XX\BKICLOC.BXX to \DBAMFG\DEFAULT\BKICELOC.B
 
In the "B" company, run DE-I-A and make sure there are no errors.  You may need to add Item classes to company "B" if company BXX has classes that don't exist in B.  Once the Error report is clean, use DE-M to transfer the data and it will append to the existing data.