The key thing to understand is that WhatsApp has no contact list of its own, it reads your phone's contacts, so the real task is getting the Excel contacts into your phone, after which WhatsApp sees them automatically, which reframes the whole thing:

Why there is no direct import: WhatsApp shows you which of your phone's contacts also use WhatsApp, drawing entirely from the phone's address book rather than maintaining its own, so there is no import to WhatsApp step, only import to your phone's contacts and WhatsApp reflects them. This is the reframing that makes the task straightforward, since importing contacts to a phone is a solved problem.

The path, Excel to phone contacts: save the Excel sheet as CSV with clean headers including a properly formatted phone number column with country codes, since WhatsApp needs full international format numbers to match, then get that into your phone's contacts through the account it syncs, for Android importing the CSV to the Google account at contacts.google.com which syncs to the phone, for iPhone the CSV to vCard route through Google or iCloud covered in the iPhone contacts threads linked alongside. Once the contacts are in the phone, open WhatsApp and they appear, WhatsApp users among them showing as messageable.

The phone number format detail that decides success: WhatsApp matches contacts by full international number, plus country code and the number, so the Excel phone column must have numbers in that format, +1 for the US and so on, before import, since a number without its country code will not match a WhatsApp account. Fixing the number format in Excel first, a formula adding the country code if missing, is the step that makes the contacts actually connect in WhatsApp rather than just sitting there unmatched.

The refresh and verification: after the contacts sync to the phone, WhatsApp may need a refresh, pulling down on the contacts list in WhatsApp or reopening it, to re scan the phone's address book, then the imported contacts with WhatsApp accounts appear. The whole task being import to phone rather than import to WhatsApp is the insight, with correct international number formatting being what makes WhatsApp recognize them once they are on the phone.