Yes it is possible. There are no direct PPCOM APIs for doing this. You need to use the low level PPCOM APIs DAP_WriteIO which takes the address and data as the parameters.
The register addresses can be obtained from the Register TRM.
There is an example app developed to write into the SFlash which you can use as reference. It can be found in the path: "C:\Program Files (x86)\Cypress\Programmer\Examples\Misc\PSoC6-BLE2-SFLASH-Update\"