Currently, there is a single command of RadRichTextBox - SaveCommand, which does indeed invoke the save file dialog.
If you wish to mimic the behavior in MS Word where there are two options - Save (to the current document instance) and Save As (which invokes the save file dialog), you can create a custom command that implements the desired behavior. There are several forum threads that show how a custom command can be created and bound to the RichTextBoxRibbonUI, for example this thread
or this Silverlight thread
(the code base is the same).
I hope this helps! Do not hesitate to contact us if you need further assistance.