Before we get down to brass tacks, here is a cute open letter to a mouse like yours -
http://rocksinmydryer.typepad.com/sha...
Here are links to several previous similar questions with answers -
http://answers.angieslist.com/Mice-fr...
http://answers.angieslist.com/mice-re...
Basically - yes it is possible, not very common for them to actually get INTO the reefer. Ways in:
1) if you leave it open for a period of time putting away groceries or getting food out, and he just happens by at that time and jumps in
2) deteriorated door seals - rarely they will even chew through a seal to get in, though likely only if not a very clean seal so it has food tasste on it
3) evidently a very few brands have freezer drain tubes (that come out at the defrost drain pan underneath) large enough for small mice to get up into, then through the cooling air duct in the back down into the reefer, if they make it past the fan without getting jammed up/injured
4) chewing through insulation around wires at back/underneath reefer, which can potentially get them into the inner recesses of the reefer and ultimately to the air circulation openings
5) chewing through filters/fabric screening on the reefer/freezer air vents
6) I heard (through contractor grapevine, so consider the source, but I had no reason to disbelieve it) of one case of a good sized rat going in through the ice dispenser - actually seen jumping from a countertop to the ice/water dispenser shelf, then up through the ice discharge opening into the reefer