I got the SPI flash working! Connecting SPI flash to the SMD09 give you a ton of additional memory to store data (e.g. for data loggers). So far I used a Macronix MX25L1005 1MBit (or 126kByte) flash. I found that on some old PCB and recycled it. There are a lot more SPI flash available, they look like they are compatible. Will test with a desoldered SPI Flash from an ESP module next. Here is a picture of my setup:
The connections are quite simple: Power, GND and for wires for the SPI bus (MOSI, MISO, CLK and !SS). The logic analyser on the right is also connected to the SPI bus for debugging. I will upload the early stages of the used code to github also.

Update: It is also working with a 25Q40 flash desolderd from a ESP-01 module. looks like there is a usable standard way to communicate with SPI flash.