Here's how I have accomplished this. First, in the actor that has the VI you want inserted into the subpanel you can add a VI Refnum to it's private data. In Pre Launch Init obtain a static reference to the VI to be inserted and bundle it into the class data. Then create a VI for your actor that takes a subpanel reference as an input, and then wire that reference to an invoke node, select Insert VI, and wire in the VI Refnum from the class data, and make a message for that VI. In your UI Actor, when the event occurs that you want the VI to be inserted on you can send the message you just created with the reference to the subpanel in your UI as the input.
You can also use the example below as a reference. It is slightly more complicated as it uses protection proxy classes for the references which may or may not be necessary depending on your situation, but the same principles I described still apply.
https://forums.ni.com/t5/Actor-Framework-Documents/How-to-Embed-a-Nested-Actor-s-Front-Panel-into-a-...
