How do I add an extra note to my measure?

Hi.

Sorry, I've tried to find the answer but just can't find it.

I'm working on a score, i use 3/4 measures. But at just one point, I wish to add one extra note, without changing the measure.
I know for example, that the Pilgrim's choir have that. If someone knows that, you might understand what I mean.

I have added a picture for you to see what I mean, as well.

Thank you for your support.

P@ric.