This is currently not possible.

You can upvote this idea:

https://ideas.nintex.com/ideas/IDEA-I-6078

In terms of reusability, there is an ‘execute another rule’ action that you can use to execute a rule that you already have.

Or you can create an unbound rule (a rule without events, just conditions and actions) and use the ‘execute another rule’ to call this unbound rule when you want to re-execute that block of conditions/actions elsewhere.

Thanks!