Yes, all 3 of these things are possible if you export your data from Excel to a CSV file, and then you use the CSV import extension.
Upon importing your data, be sure to choose the option to “merge with existing records”. Note that it will only let you choose ONE FIELD as your “match field” for merging, so be sure to choose something that will always be unique, like an account # or a phone number or an email address:
I have installed the app extension so I understand what you mean now. My new problem is that instead of adding a second value in some of the fields it has replaced the original values.
My match field is “Company name”, other fields (“email address”, “contact name”) are set up as multiple select so I was hoping to get a second entry merged to those. Instead it has replace the original with the new value.