Currently it is possible to import the private key after setup (sans some problems in DeltaChat fails to import keys), but only after generating a new key. As a result, this key cannot be used to decrypt old messages.
This feature proposal is about adding an option to select the key during setup in advanced settings. This can also be a place for the generated key type option requested in Allow RSA 2048 and 4096 bit keys, a single setting with multiple options:
- Generate ECC key (default)
- Generate RSA key
- Import existing key
Currently fetch_existing_msgs setting is disabled (fix: disable `fetch_existing_msgs` setting by default by link2xt · Pull Request #4423 · deltachat/deltachat-core-rust · GitHub) but we may consider enabling it again as an option, e.g. if the user has the key backed up and wants to get old messages from the server.