Please help us help you and fill where relevant: "others" field 
Your LimeSurvey version: Expert version 
Own server or LimeSurvey hosting: Limesurvey 
Survey theme/template:
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 it possible to add an "Other (please specify)" option with an open text field to a single choice (radio button) question? If yes, how?

Yes, it is.
Please read the manual here
[url] www.limesurvey.org/manual/Question_type_-_List_(Radio)/en [/url]