There is no easy OOTB answer to this. The fully formed way is to write a set of Java connectors to address the use case and leverage them. If you want more of a hack, it turns out if you create a file with an html table in it and name it with an excel extension, it will open properly in excel. So knowing that you could write code that uses the System Toolkit write a text file with that content to your server, then use the OOTB e-mail connector to generate / send the e-mail. This isn't an ideal solution and requires a bunch of be-spoke code, but it should work.
BP3 has toolkits that implement the java solution using open source packages to answer both halves of this problem. One is a toolkit that can generate a multi-sheet excel spreadsheet from lists of Business Objects via configuration data. The generated excel can be written to the file system, attached to a Process Instance, or returned as an ECMDocument (or whatever that is called in the IBM ECM Toolkit).
Using that last option, the BP3 E-mail toolkit addresses many of the short comings of the OOTB e-mail toolkit, handling secure server connections, servers that require logins, and, importantly for this answer, use a ECMDocument for file attachments.