SERout is configured as an output and NOT tri-stated with the LE (chip-select-bar) pin.
That means, it works fine to connect the SERin and CLK to an SPI bus.
BUT DO NOT connect SERout to the SPI bus, as it will either get fried or fry something else, unless it is the only thing on the bus.
Use a tri-state buffer to isolate the SPI bus from the SERout output--like through an SN74LVC1G125DPWR, and connect its OE-bar to the HMC629A's LE. If you typically use configurable logic gates, a 74LVC1G99GS-115 also works.
I would recommend to ADI:.
1. Update the datasheet and expressly point this out.
2. Change the part so that SERout is tri-stated so the part works alone and can be connected to an SPI bus without helper chips.