Don't think you need a custom plugin. What you may be able to do is the following:
Create the fields you wish to import using CB Field Manager.
Next, download the plugin called CBJuice from Joomla extensions directory:
extensions.joomla.org/extensions/extension-specific/community-builder-extensions/community-builder-activities/2613?qh=YToxOntpOjA7czo3OiJjYmp1aWNlIjt9
Install the plugin and make sure it is published.
Next, export the entire database using CBJuice. It will save the file as a CSV (Comma Separated Values) format which MS Excel will be able to open natively. You might have to use the Text to Data feature in excel to get the file to layout correctly.
Now open your other excel spreadhseet and copy and paste the columns and rows of data from that into your imported users.csv file.
Save the file and then use CBJuice to import your new data back into the database.
Hope this helps.