Import customers from 2.8

I have two nop 2.8 sites, now my client wants to merge both the sites to the third nop 2.8 site, I have exported the customers to excel, not sure how to import them to the third site, any help on this please??

It would require custom coding.  Also, note that the single address info in the export is from the customer registration (if it was enabled on your prior system) - the export does not include other Addresses (entered during billing/shipping/etc.).

We can manage without address, how do we import what ever we have in export file?

It still requires custom coding.  Create a plugin, or easier, modify ImportManager.cs, and add bbutton in
\Presentation\Nop.Web\Administration\Views\Customer\List.cshtml
and add method in CustomerController to handle to handle button post.

Look at how XLSX import is done for products:
\Libraries\Nop.Services\ExportImport\ImportManager.cs
        public virtual void ImportProductsFromXlsx(Stream stream)

Look at how customers are created in
\Presentation\Nop.Web\Administration\Controllers\CustomerController.cs
        public ActionResult Create(CustomerModel model, bool continueEditing)

I am not good at these things, isn't this a very important feature of any eCommerce platform?

You can pay someone to do it for you.  e.g. Solution Partners

Can't afford paying at the moment, can't I import the excel into db directly through sql management studio?

Yes, but it would require some work - massage data into correct format (e.g. true/false vs. 1/0), default values for fields not in export, etc.  And you would need to align fields by adjusting the SELECT statement in SSMS Edit tool.

And then, insert Customer Role Mapping records for each giving them Registered role.

Also, forgot to mention.  Passwords are not in export, and they are encrypted in the db. So you would need to work that out too.