Can I re-cook potatoes stored half-roasted in the fridge?
Theoretically roasting them more should cook them further, however after cooling and drying in the fridge they may not come out with a very nice texture. Try microwaving a few of them before you roast them, if microwaving cooks them to they are done and you still want to eat them afterwards then roasting is a good option. If microwaving them doesn't work out roasting isn't likely to either. 

When roasting potatoes I prefer to par-boil them before roasting, it really gives a much better result! See my answer to this question for some more details.