Yes, FSB Pro can add to an open position and a new same-direction entry signal.
FSB Pro can also reduce (partially closing), close, or reverse a position if an opposite entry signal occurs.
However, FSB Pro does these operations on MT4 and Netting MT5 accounts. it does not work on Hedging MT5 accounts.

EA Studio and Express Generator use one market for all indicators.

We can create multi-market strategies manually in FSB Pro. It allows us to set a custom period or symbol for a particular indicator.

Some traders find such strategies work well for them. However, the complexity of the code makes the MetaTrader testing very slow. That's why we decided to have a more straightforward strategy structure in EA Studio.

If I have understood correctly, FSB is capable of multiple time frame strategies as well as entering and exiting positions intra-bar (e.g. not just at the open of the next bar). Is FSB capable of more complex trade management techniques like adding to existing positions, and partial closes?

We can create multi-market strategies manually in FSB Pro. It allows us to set a custom period or symbol for a particular indicator.

Some traders find such strategies work well for them. However, the complexity of the code makes the MetaTrader testing very slow. That's why we decided to have a more straightforward strategy structure in EA Studio.