Is there a way to Import from Office 365 to Roundcube?

I have a client who another service provider charged them an obscene ammount of money for 4 office 365 accounts which they did not need.  $2k/y for only 4 email accounts...  They only use the email feature and thats it.

Each email account has between 5GB to 10GB of emails in them.

Is there an easy way to transfer this over to CWP?

I think this should help.
https://www.namecheap.com/support/knowledgebase/article.aspx/9403/2226/how-to-transfer-emails-to-cpanel-mail-account-using-outlook

Just replace the Cpanel email settings with CWP email settings

That doesn't work to well for large email accounts.

Almost 10GB each with many folders and subfolders.

Need a quick and easy solution.

Try to export in mbox format and then import it into roundcube. There is no easy and faster way. You may need to breakup your inbox into multiple folders before exporting as and export each folder seperately so that a single file does not hit the php file upload limits in cwp