How to add new layer to the opened file by the action/droplet?

Can you try one more time to explain to us what exactly you need.

It seems that you have open file. You are running action to do something to that file, right? Then you are running another action after manual adjustments to export file as JPEG, right? What is actual problem? Are you looking for option to pause action to do some manual adjustments or for option to play another action using action step? 

I do not understand what is doing word droplet in your explanation if you are running second action manually in your process to export file. Are you using droplet to place file in Photoshop or something else is in question?

Update: it seems you are looking for way to place file in your document. It also seems that you want file placed at some exact position in Layers panel in existing group? There is a way to place file using File > Place Embedded/Linked command which can be used to place some exact and always the same content or to open Place dilogue to choose file to place. The more difficult is to place file at some exact position in Layers panel. If that position is static, in other words always the same position it is doable but you must select bottom most layer, for example, using action before placing file then to move it on position for what is recommended to use keyboard strokes what will record movement for one or more places in Layers panel. If my guess is right then the answer is positive it can be done but with some assumptions that you are using always same file or files with the same layer count or order what is important to be able to move placed file into some specific position in the Layers panel.

Droplet are basically an automated batch process therefore should not contain interactive manuals steps. Actions should have no Problem creating a new layer containing a image in a particular jpeg file.  If the action need to determine which jpeg file the layer should have that action step would need to use a Photoshop Scripts for actions can not use logic to develop file name the need to use.   If you need help with an action you want to create a droplet with you should include the action in your post so we can see exactly what the action does.  Action need to be design correctly to be used as a droplet