It is possible, but only if the race is razor-close or if Johnson receives considerably more attention than presently.
Recent polls show candidates offered as “other candidates” in presidential choice questions are getting one or two percent. Gary Johnson, when specifically offered as an alternative in Colorado, has received from three to five percent in recent automated polls.
Unfortunately for Johnson, he will only be listed with a dozen or more alternative party candidates in Colorado. He has generated little publicity or interest and has no campaign. When his name is mentioned in poll questions, he tends to draw from younger voters who are both Barack Obama and Mitt Romney supporters. So, unless the presidential race is down to a couple of thousand votes, Johnson is unlikely to affect it.
Recent national polls offer respondents a generic alternative category along with the major party candidates. “Other” has been receiving one percent of the vote (ABC News/Washington Post, Sept. 11, 2012). Gallup reports minuscule support for third-party candidates nationally when identified by name and party. Gary Johnson of the Libertarian Party received one percent.