If the plants are dormant, then the fertilizer shouldn't cause any issues. Miracle Grow or Scotts potting medias all have fertilizer and that is what I have used when potting up clematis to overwinter in their pots. Just leave the pots outside until the plants are dormant due to them having several frosts. Once that happens and the temps. start staying consistently cold, move the pots into the garage assuming it is not heated. An ideal temp range is so that the pots don't freeze but stay around 35 to 40 degrees. Luckily for me, I have had success just allowing the pots to sit on the ground outside over the winter. Another option is to bury the pots in holes in the ground.