Don't use /skip in this case, use /branch.
<surveypage demographics>
/ branch = [if (radiobuttons.gender.response=="female") surveypage.femalefollowup]
/ questions = [1=gender; 2=age]
</surveypage>
<surveypage femalefollowup>
/ questions = [1=stuff]
</surveypage>
<surveypage funnybunny>
/ questions = [1=stuffpuff]
</surveypage>
<radiobuttons gender>
/ caption = "You are"
/ options = ("female","male")
</radiobuttons>
<textbox age>
/ caption = "Your age"
</textbox>
<textbox stuff>
/ caption = "Because you're a woman, write stuff here:"
</textbox>
<textbox stuffpuff>
/ caption = "and now press the back button to get back to the demographics page…unfortunately it will lead you to the femalefollowup page as a male and female"
</textbox>
<survey mysurvey>
/ pages = [1=demographics; 2=funnybunny]
</survey>