Yes, even if you recorded with the mouse clicks off, you can add them as an effect, post-recording. Just go to the "Cursor Effects" section, choose either the 'left click' or 'right click' category and drag the 'click sound' effect to the clip. I drag it on the 'screen recording' clip, not the audio one. I think the screen recording track records when the clicks occurred. I just tested this in Camtasia 2020 and it works.