You can achieve that with jQuery modal, but I would look into a more user friendly solution. What exactly are you trying to do?

I have a receipt form that opens in a lightbox window.  Within the receipt form, the user can select the merchant, select a storage location for the physical receipt, and select existing items linked to a line item.  Each of these supporting selections opens in its own lightbox window.

Each supporting selection works nicely when launching the main receipt form as a regular page.  What options might there be for launching the form in a lightbox window?

I think an autocomplete is a more suitable option. Take a look at elgg_tokeinput on my github.

I considered autocomplete, but decided against it.  I use the selection forms to do more than one thing.  They provide options for the user to set metadata on a selection.  They also allow the user to browse and filter the selection options on criteria. 

I'll check out jQuery Modal.

Any other thoughts?

Then jQuery UI dialog is the way to go, since Elgg already loads jQuery UI. https://jqueryui.com/dialog/