FUB (Function Block) symbols are very useful in a "top-down" hierarchical design entry process, where block symbols can be resized and pins added dynamically within the schematic.
However, if you need to work in a bottom-up fashion, the Tools -> Generate Symbol menu generates a standard, static symbol that cannot be resized, and pins can only be added in the symbol editor (or re-generating the symbol).
The attached script provides a new Tools -> Generate FUB Symbol menu that generates a FUB symbol from the port instances on the current schematic sheet. The script opens a temporary schematic, places a fub symbol and wires a net/pin for each port. The temporary schematic is deleted, and the FUB symbol is saved into the Local Symbols partition. The newly generated FUB symbol can be placed on a schematic, and can be resized and have new pins added by connecting a net.
Load the script on startup by adding a reference to the [viewdraw] section of the scripts.ini file: