Here is the first version of my script.
The script is not pretty, but you only need it once to migrate all your data. 
Possible problems:
- I am not sure if the defined timestamps are correct to read as much data as possible
- I am running influxdb as a docker conatiner on my raspberry pi.
After importing four items with ~1000 values each, the container uses ~80% of my pi’s memory.
After restarting influxdb, the memory usage gets normal again.
If you run the databse on a little system like the pi, I recommand stopping other services for the time being. My system got almost unresponsive last evening.