Hello Juha,
Yes, what you are looking to do is possible. For example for a product like the ADAU1701, you can bypass the stereo ADC and four DAC outputs and use it as a "digital-only" interface to an external codec. Or use a combination of ADC/I2S inputs and DAC/I2S outputs.
From SigmaStudio perspective, it is easy to read from either analog I/Os or digital I/Os by connecting to the correct channel assignment within the I/O cell block. The schematic view and channel assignment tables are shown below. The bottom channels are reserved for the ADCs and DACs, and upper channel assignments are reserved for I2S or TDM channels.
-JT