Lead Import with additional data for existing leads

Lead Import with additional data for existing leads

We are having difficulty importing multiple data sources with each having different data fields for leads where some of the lead records are already in ZOHO. 

For instance, we have 100 Leads already in ZOHO, and have 3 files to import. All files have FName, LName, Company and LinkedIn address (along with other data fields), but not all have email address. If we could have the unique key as LinkedIn address instead of email, it would work great. 

As it is, it appears we would need to dump everything out of ZOHO, programmatically merge all the files together, and then re-import. An enormous task given the number of external files we have, etc.

We are noobs to be sure, but any insights would be greatly appreciated. We'd hire the expert help as well. 

Thanks.