Your list doesn't have to be in an excel spreadsheet, you can create a list just by typing in the contact info you have.
Create a list by clicking the green plus button, give your list a name and then select the Add Manually option on the right side of the page.
We have all the basic fields you'll need to keep your contact info updated, plus any custom fields you've already created. Don't worry about filling in everything, just fill in the info you have. At the very least you'll need an email address, and then click Save & Exit if that's the last address, or click Save & Add Another if you have another one to add.
You can create a list like this if you like, but you can also use this option to add an address to an existing list as well, no matter how that list was created to start with. To add one address manually click on the list you want to add it to, then click the green plus on the right side.
This will bring up the Add Contacts screen, click Add Manually and fill in the info for your new contact.