trouble with using waterproofing on DWR is that you can plug up the pores that allow the jacket to breathe, so you just get a lot of condensation inside and get wet anyway

I'd check the Marmot website and see what they recommend - I'll bet there's an answer in the FAQs, but if not, send them an e-mail and ask what they recommend for thier product - might be a safer way to go.

I've never treated any of my rain jackets, just eventually replace with new ones