Many MTH tenders have the provisions for the roller, in that case it's a "bolt on" installation. If they don't have such a provision, you can normally accomplish the same task without a great deal of trouble, here's a couple of examples I've done. I use either fiberglass sheet or Styrene sheet to build the insulator stack for the roller. CA adhesive is used to build the stack.
I've done this on a lot of conversions as well as just retrofitting them for existing locomotives.