There is a requirement to import the additional contacts into dynamics 365
So i have opted to use the OOB functionality, i.e Template from dynamics 365.
Goto Settings > Data Management > Template for data import and select the entity you want to download as a template.
Please find the below diagram:
Then download the template and add the records you want to import into dynamics 365.
Import was partially successful with 977 records failure because of the lookup reference could not be resolved.
In my case the additional contacts have companyname but inside dynamics 365, there is no accounts with those name, so the lookup could not be solved.
So i have created the new accounts in the dynamics 365 and reimported the failed contacts and the import was sucessful.