Creds are stored in /storage/.cache/connman/<service>/settings .. where <service> is something unique based on the SSID used, etc.
However the file is "managed" by ConnMan so you need to stop the connman service to copy the file over and then restart connman. The challenge will be that when you stop or restart connman you may be disconnected. IMHO it's much easier to just re-enter the passphrase; and if you've chosen one that's long and complicated do it via SSH:
connmanctl
agent on
scan wifi
connect <service>
<enter password>