Reader can not import button icons. Reader can only use an icon stream.
Acrobat can import a PDF for an Icon, but you would need to know where the pdf file is.
Much easier and faster to create an Icon stream from a text object within the PDF or script file.
There is a tool for creating Acrobat toolbar buttons called AcroButtons, it includes a tool for converting any standard image file into a icon stream object. Actually, what the tool does is create a code template for an Acrobat Folder Level automation tool, where the button is a front end for the tool.
You can also get detailed info on writing your own code for creating button icons by joining the pdfscripting.com web site and reading this article: http://www.pdfscripting.com/public/department35.cfm