1. Place PDF in InDesign
2. Make a new CMYK spot color, defined 100C 100M 100Y 100K
3. In Ink Manager, find spot color of the PDF in the list.
4. In the ink alias for this, pull down and select the spot color you created in step 2
5. Still in the Ink Manager - beside the spot color you created in step 2, click once on the spot icon on the left to change it to process
6. Output a new PDF from InDesign
Once you have the InDesign file set up, all the work is done for any future PDF files you place. InDesign simply takes the spot color in the original PDF (ie Pantone 293 C) and maps it to your 100 CMYK spot. The 100 CMYK spot is in turn forced to process by the ink manager (the ink manager can be accessed in the flyout in the swatches panel)
I would bear in mind that the presence of any colour management transformation afterwards will defeat that (as it is quite unlikely you're going to fin an ICC with an 400% TAC).