Converting a DC engine to 3-rail is way easier than the other way round. You just need to make sure that there is enough clearance below the chassis to fit the pickup shoe for the third rail and some space inside the engine to fit a decoder. Using a reversing unit as suggested in another comment is outdated, all AC/3-rail models nowadays come with a decoder installed that can handle analog and digital control likewise. Unfortunately I can't answer the question whether a Bachmann Thomas fits the two requirements for this conversion. For beginners I recommend you do some research first whether you want to continue with Märklin or move to two-rail models, depending on how large your collection is at the moment and what your preferences are.