We have a general guide that covers migration. This might not solve your immediate problem, but could also offer you some insights as well on a broader scale.
Connectors come to mind for this use case, but I am not 100 percent certain they fit your use case.
https://fusionauth.io/docs/v1/tech/connectors/
Or possibly using the user API to ingest from your SAML provider beforehand but setting up dummy passwords (seems less secure, however 🙂 ).
https://fusionauth.io/docs/v1/tech/apis/users/
Let me consider this further and I will post back any thoughts.
Thanks,
Josh