have about 450 leads in a spreadsheet in CSV form. Want to upload/add them to my contacts

Using windows 10, with Windows Live Mail 2012.

Have about 450 leads from a service that I used and have downloaded them in excel spreadsheet in CSV format.

Can I import them into my contacts?

I have some 6000+ contacts currently.

If I add/import these 2 files into my contacts will it delete or affect my existing contacts? I don't want to lose them. I simply want to find an easy way upload these leads into my laptop (Windows Live mail 2012) without typing them in individually. I also don't want to lose all the existing contacts nor the groups i have currently.

I think I have to organize the spreadsheet's columns so they match up with the same order if I export the existing6000+, correct? I basically only want the names, 1st & last, the e-mail address, the phone number and I have assigned them a name (column) that will serve as 'company name' which I can then go grab the group and put them into a category under contacts? That appears to be the "S" column in the spreadsheet.

Do I know what I am doing?

I basically want their first and last names, phone number, e-mail and then assign them a "company name" that I can search and pull them into a group/category, together.

Thank you