Importing bulk Leads, Opportunities and Accounts - How get them do the relate after the import?

I would like to import roughly 500 leads into SuiteCRM. The leads will be exported from an old estimating system and have been formatted to suit the lead import spreadsheet field names.

Of those 500 hundred leads we have around 350 that would need to be converted to Opportunities, Accounts and Contacts. If I manually create a spreadsheet for the opportunities and import it into SuiteCRM how would they tie up with the corresponding lead as I don’t know what the Opportunity ID would be. Likewise for importing the accounts and contacts - how would they tie together without knowing the IDs?

I would be open to other suggestions on how to bulk import a bunch of leads that have been converted.