You have to use the 0-10V two channel output, exposed as audio connector. You'll have some ancillary circuit and a dedicated power supply for the dosing pumps.
The pico board was designed to support a pico reef tank and the intended use case did not involve doser. It was only equipment, temperature and light. These were my suggestion . @theatrus added the ph circuit, a board specific MCU and bunch of other goodies in there, but still nothing targeted for dosers, per say.
But as I was mentioning, it should be possible to use the 0-10V pwm to control a purpose-built doser.
If you only want on/off control, you can use the digital outlets to drive an external FET and a doser pump that way.