Hi Electro ns,
since SSI is a synchronous type of serial transfer and you generate the master clock it can be generated in software if you meet the timing of the device connected. The general SSI description can be found here:
https://en.wikipedia.org/wiki/Synchronous_Serial_Interface . There are some differences from MFG to MFG and you need to have the timing requirement of the device you use. There is a setup and hold time, maximum clock speed and time out at the end you need to considered. Also no clock spikes are tolerable. At the beginning of the cycle you could check in software if the data line is high.
Enjoy your design work!