Sorry, that specific method is not implemented in the Ruby client. (Edit: It is, see below). You can easily make the call yourself through the underlying Podio.connection object. Take a look at any of the models in the github repo to see how it's done.
I would not recommend using the Importer area for importing an Excel file, but unless you need to build something very flexible with mapping etc. - just like we already have inside Podio. It's quite complicated to setup this mapping and make the import work.
I'd recommend you parse the Excel file yourself and simply make calls to Add new item with the data you wish to insert. That should save time and give you much better control at the same time.