Ye, say you need 18V instead of 20V. A step down does the trick there. In other cases such as Li-ion chargers, they expect a constant current supply which fpx board doesn't do. So a DC-DC module with max. current adjustment needs to be added in-between to accommodate.

In short, it does take some understanding of how the device you're modding works and what power draw characteristics it has, before converting it to use USB-C power.