How to: Input Die lines from a PDF into a layer.

The PDF may be layered, so there is a possibility you can simply switch on/off the layers you want.

But also as is, there may be no need to modify anything. Just put the whole graphic on a Indesign layer and use that as a reference when you do your design. Your print service provider will do what is necessary.

The best advice for now is what Scott Falkner said:

But again, communicate with the printer. This is the first and most important rule in printing.

Were you supplied the dieline by the printer? If so then ask them what they prefer. They might prefer you place the PDF in InDesign as is and their prepress department will take care of the rest. Always talk to the printer and make sure you’re not making any assumptions.

What I would probably do is open the PDF in Illustrator and select just the dieline. I would copy the dieline and place it in InDesign on its own layer; this would be the topmost layer in the file and only contain the dieline. I would stroke the dieline with a custom spot colour, preferably an ugly one that looks nothing like any colour in the file. The colour is not important, what I want is contrast so I can easily see it is there. In the Attributes panel I would make the dieline stroke overprint.

Doing that I would have a dieline that does not affect the printing of any other colour separations and prints on its own plate. The print shop would handle generating the die.

But again, communicate with the printer. This is the first and most important rule in printing.

Did you receive instructions to have these lines in the finished document, or are you merely wanting to use them as guides? If the latter: place it in InDesign on a nonprinting layer and lock it. Draw your art on top.

If the first: then best asked in Illustrator as InDesign cannot change a PDF. True, Illustrator is also not a "general PDF Editor" but you might get lucky.