If I understand your question correctly, to add a segment to an active campaign:
1. deactivate the campaign
2. drag a segment members box to the canvas from the audience section of the left hand bar of the campaign canvas (sometimes there is a slight time lag before it appears)
3. click on the segment members box you just dragged and configure the segment by choosing whether to add hourly or when first activated, and either pick an existing segment or click create new.
4. connect your segment members box.
5. reactivate your campagin.
Your prior segment box remains untouched.
Is that what you' were looking for?