Hi,
The only way that I know is to create a blank PDF in Acrobat first. Then import this PDF into LC Designer. You will not have all of the LC Designer features available to you, but you will be able to create a basic form.
Then when you open the form again in Acrobat, the Flash Tool (from the Tools > Multimedia menu) will be available to you.
Thom Parker has an article here: http://acrobatusers.com/tutorials/2008/08/js_flash_buttons/
The trick is getting the Flash content to communicate with the PDF/XFA content. Thom uses the external interface call to fire a function. This works well with Document Level functions in an AcroForm, but I have been unable to get it to fire a script object function in the XFA form.
Incomplete sample here: https://acrobat.com/#d=HAp0kKvcWk7X5N52HxcXng. The colour picker is meant to change the colour of the XFA objects, but at the moment it does not call the function "xfa.form.mobileFlash.swfComms.setColour", even though you can fire this in the javascript console.
Good luck,
Niall