You cannot increase the position of a trade. You can either partially close it or close it entirely.
Suppose your position is 10 lots long, and you want to increase it to 12, you enterLong for 2 lots. Yes, now you have to manage 2 trades.
Alternatively, if you are using the Virtual Hedging feature (Hedge = 5, Zorro S only), you can close the original 10 long trade and place another 12 long. At the end of your run() call, Zorro will place a single +2 lot trade to split the difference. This is a very nice feature.