Add plugin code to theme… license problem?

I have a theme that would be more functional with the code from a free plugin (this one: https://www.ads-software.com/extend/plugins/press-this-reloaded). It’s premium/paid template so I don’t know what the rules are for including this, if it’s at all possible. I recommend people install it but I’d rather just have it come as a default.

Is this kosher or not? Thanks!