Pulse Interval Encoding
Are you actually sure that transceiver handles the encoding you want to use? it isn't mentioned in the datasheets.
Some transceiver (TI ones, for example) have a bypass mode (or test mode) where you can disable the digital modulator and directly drive the RF section with an external digital signal. Then you use an MCU/FPGA/whatever to create the encoding you need (usually you are restricted to OOK/CW for RF encoding).
The ST chip you mentioned seems to have a test mode for continuous transmission or pseudorandom modulation (useful for testing the RF stages) but no such bypass operation mode. There is a direct polar mode (only for TX), are you referring to that?
If you really need to use PIE I'd suggest to use another transceiver or simply ask directly to ST (good luck with that, in my experience)