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Yes, exactly - thanks, I was away from a real keyboard.
I have done that as I was in the same situation of the OP:
- did not want to bring two different I/F to the board
- I2C used by all the controlled chips
- a port extender was already needed, and hade enough free pins
- I only needed to write to an SPI-like controlled RF attenuator, at a leisurly pace.
I2C-SPI bridges chips exist, such as the one I linked to in my post, and they are a simpler solution though not cheap.
For a cheaper way, there are small programmable devices (a full MCU as suggested by Ian.M might be too much?) that fit the bill.
E.g.
https://www.renesas.com/us/en/document/apn/cm-334-bidirectional-i2c-spi-bridgeStill, if a native I2C solution can be found that's the best way.