Q:

How can I lookup an account ID for a Case using a field I am importing that is not the account name?

I have to import a history of cases in Service Cloud and, sadly, the account names I already have in Sales Cloud and the ones in the import don't match.
I have a Client ID though in the Account object that I have available in the import as well.
How can I use it to fill in the right Account for the Case object upon import or even later (in case I have cases to be imported that don't have a Client ID)?

A:

Mark the field Client ID on Account as an External ID. 
Then use dataloader to insert or upsert the Case records. The dataloader UI will ask you which foreign key on Account to match with, and which field on your Case upload file to use for that match.

