if she's overweight already i'd maybe give her something like hifilite or equivalent chaff with a vit and min supplement in just so she's got ehr vits and mins-if she's getting grass thru the day she shouldnt need anything else really!its more to make her eat the supplement than give her any calories

i'd also double up your haynets on a night to keep her eating longer and if she's not already switch to hay rather than the higher calorie hayledge!

also use as few rugs as possible-she'll use energy to keep warm (i dont mean naked in the snow-just generally dont fuss 2 much about keeping her toasty) and turn out as much as possible so she can exercise herself to a degree