You can just use the mat - however you really should put it on either a rheostat or a thermostat as they can overheat and burn the snake or crack the glass.
I've found most balls switch readily from mice to rats - the ones I've had that are strick mouse feeders have had other feeding "issues" as well and I'm proud to say I've gotten those switched over too. Best way to switch is to toss a rat to them and see if they take it (most of the time they will). If not - scenting the rat with mouse works or depending on the size of the snake you might want to toss in a small mouse and follow up with a small rat (keep your meal appropriately sized). Sometimes it takes a couple of tries with rats but in all honesty very rarely do I have problems getting them to make the switch.
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Tosha 
"Nihil facimus sed id bene facimus"
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Anakin Skywalker)
0.0.1 Green Tree Python (Verdi - yeah I know but my kids love the book)
0.2.0 Feline (Pippen and Pandora)
0.0.1 Desert Tortoise (Pope John Paul aka JP )
2.2.1 Fish (1,2,3,4)
0.0.2 frogs rescued from pool skimmer