Read this post first:
Can we create New Button in Word Perfect toolbar using Perfect Script?
Depending on where you want your "button" to be placed, you would use ButtonBarAddItem() if you want the button added to a menu.
There are other options to insert menu items as well, but they don't all work.
If you are using .NET and want to add a button to a dialogue box in WordPerfect, you would need to implement the PerfectScript.Script interface to create the box and then add the button. But that route seems overly complicated, since you can very easily create a modeless form that contains an instance of WordPerfect into which you can insert Visual Studio's myriad controls, including buttons. If this is what you mean, look at this post:
WordPerfect Tools--Using Forms to Automate WordPerfect
Ric Gaudet