For a custom object , we have a lookup field Account which will be holding the id of the Account which the user will be selecting .
We need to import data from a 3rd party where the Account field is text .
How to insert such data?
Data Loader Option
Simple Option use Import Wizard
The Data Import Wizard will allow you to match Salesforce fields using text values, e.g. account name, rather than the Id. See the image below though there I am matching user rather than account name the principle is the same.
Setup>Administration>Data Management>Data Import Wizard
This will work better if you have less than 50,000 records to import or can batch them up.