Hello Tim,
If the "pop in" is a word that you have predetermined, then it can be
part of the animation on the slide. One such choice would be to fill a
text box with the word or words, right click on the text box, select
custom animations, and then the entrance of you liking. When the slide
appears the text box will be hidden until the slide is clicked, at a
time when you need it.
If you are needing to add text from the audience, another approach will
do that for you. Post back if you need more options.

I used the method above for years but found it wasn't easy to line up the
text boxes with bulleted text. One day I dreamt up this cunning plan!

Type all the text into a bulleted list including the pop ups.
Highlight the pop up text and change the colour to = background colour
Now add an emphasis animation to change the font colour to = the usual font
colour