Add content to template

Hi, please I need your help!

I´m using a free template called Modality and the thing here is that in the theme editor I can only change colors, text, add and quit elements for the static site… but the BIG question here is: How can I add an external plugin to the theme??… I want to add an image slider to the home of the theme but I don´t know how… I´ve been looking for a solution everywhere but I dónt find anything. Please help