If you mean on the "buttons" created by a passage like the StoryMenu special passage, then yes—though, how precisely depends on exactly what kind of markup you're using. If you meant something else, then you need to be specific.
For example, using their generally suggested method of an <i> tag:
/* With a simple passage link */
<a data-passage="Passage Name"><i class="fa fa-pencil fa-fw"></i>Link Text</a>
/* With the <<link>> macro (watch your quotes) */
<<link '<i class="fa fa-pencil fa-fw"></i>Link Text' "Passage Name">>…<</link>>