The easiest way is probably to configure your PCF8575 as an expander, i.e. on zones 9-24. The reason is that this is already in the design of an older version of OS expander. The hardware design files are here:
https://github.com/OpenSprinkler/OpenSprinkler-Hardware/tree/master/Expander/3.0/DC
look for the exp30dc one (you will have to install EagleCAD to view the files). The expander I2C address starts with 0x24 (i.e. A0=1, A1=0, A2=0). You can add up to 4 expanders (0x25, 0x26 and 0x27). The particular design of expander 3.0 uses a lot of components (it uses a PNP transistor per channel for logic inverse, because PCF8575 has persistent internal pull-up so it defaults to high on each output, and the PNP transistor reverses that to default low).
So the short answer is that yes you can connect PCF8575 as an expander. This way you existing 8 zones are usable and you get 16 additional zones.
If you can get a PCA9555 board, though, the circuit can be a lot simpler. That’s what expander 3.1 DC is based on.