I had never heard of PERRLA before, so I'm glad you asked.
Is there a macro or a PERRLA version for Nisus Writer Pro?
I'm not aware of a macro that does what PERRLA does, and according to PERRLA's web page, they only offer PERRLA for MS Word.
I found PERRLA that is an add-in for Word.
It used to be a Word add-on, but now, due to sandboxing, it's a separate database application.
http://www.perrla.com/HelpCenter.aspx?_ ... 1491143374
Nowadays the new thing that colleges and universities are crazy about is the APA writing system!
It depends entirely on your field of study. The APA style guide is used preferably in the social sciences. — What is your field of study?
I was wondering if there is anything similar that can be added (like a macro) to Nisus.
PERRLA does merely two things.
 It formats the document according to the APA style guide, and
 it stores references in a database, and inserts citations and creates a bibliography based on those references in the APA style.
It seems their latest version doesn't offer any other style for the Mac; on their web page they say: "MLA has not been added to the program yet, but it will be soon."
Well, the first part can be easily accomplished in Nisus with Styles. Since the floating Styles palette can be dragged around and placed wherever you want on the screen, formatting the document as APA can be done with a (single) mouse click. It's actually a piece of cake. — Once correctly formatted, one should save the document with the styles as template and delete styles that are not needed.
A macro could be written for the second part using a repeated "Prompt Input" command. But the question is whether that would make much sense. Where are you going to store all the references you accumulate over the years? In another Nisus document? That would soon become unwieldy if you have thousands of references. Where are you going to write your notes and comments? In that very same Nisus document?
Entering references manually is also a thing of the past. It not only takes too much time, but it's also prone to typos. A good reference management software will fetch every piece of information you need from online databases or from the library of your choice and format it automatically in whatever style you want.
When choosing the right reference management software one should perhaps first answer the question whether one intends to cooperate with other colleagues who are working on the same topic or not; some reference managers will allow you to create online groups and actively share bibliographic information with other group members.
If you haven't done it already, you might want to try out Bookends
(free for the first 100 records and integrates very well with Nisus Writer Pro), or Zotero
(free) and Mendeley
(free). For all three there is a large number of introductory YouTube videos available.