If the button opened a new window with a data entry layout it could look very much like a popup depending on how you design the graphics and place the window.
Additionally, you could switch layouts on the existing window and still have it "look like" a popup (even though it isn't one).
When the last field is filled in, trigger a script to close the window and you'll be left where you started...
What am I missing?
It can be done with a script that uses either the New Window script step or Go To Related Records with the New Window options specified.
You have to do a few scripting tricks to make the popped up window "modal". (Keeps the user from bringing the underlying large window to the front--hiding the pop up window--simply by clicking on the larger window.) And you have to add a few extra steps on windows machines to keep the underlying window from resizing in an undesirable way if you have it maximized.
There are a number of examples here on the forum if you search for them, but they all use variations of this basic script:
New Window [//specify size, location and name of new windo in these parameters]
#Adjust underlying window so that it is as close to "maximized" as possible. Window is assumed to have same name as the file name (default)
Move/Resize Window [ Name: Get (Filename ) ; Current File ; Height: Get (ScreenHeight) ; Width: Get ( ScreenWIdth ) ; Top: 0 ; Width: 0 ]
Show/Hide Status Area [Lock; Hide]
Allow User Abort [off]
Go To Layout [//select layout for your pop up.]
#All user actions that dismiss the popup must use Halt Script to terminate this endless loop