You would have to add that Balao boat to the CareerStart.upc and Flotillas.upc files in the Data / UPCData / UPCCampaignData folder, as well as assign missions in the Flotillas.upc file. The boat is probably already in there (except the CareerStart, but just copy and paste from later in the file), so you'd just have to change the dates of appearance. You could probably copy mission assignments from one of the other boats, so that you get some appropriate assignments for where you are. Just be sure to change the boat and mission numbers to be sequential. Look at the other boats in the Flotillas.upc file for examples.
[Flotilla 1.UserPlayerUnitType 1.Objective 1]
ID= PH1Obj1
NameDisplayable= East China Sea
AvailabilityInterval=1942-01-01, NULL
ObjectiveCode= East China Sea
The "orange" text is what might need to be changed for your boat, as well as the ObjectiveCode and NameDisplayable, especially since the SubSoWesPac boats didn't go to the East China Sea until really late in the war, when based at Subic Bay.
