Sympathy Traits (RoP:F): from negative to positive?

It is possible to improve negative Sympathy Traits (from RoP:F) all the way to 0 through experience. Do they then "disappear" or is it possible to improve them further into a positive value? E.g. a character with a Wolves -1 trait spends 10xp to improve it, so the Trait value is now 0. Can he spend another 5xp to bring the Trait to +1?

It is certainly possible to reduce a Negative Sympathy Trait to 0 with XP. I've always understood that the trait become 0 (distinct from no trait at all in the same manner as MR) and can then be improved further by spending XP, but I'm not sure how officical it is.

Sure, I'd allow that. The character's supernatural weakness with wolves can be overcome with practice and even transformed into an affinity for them. Either way, the character has the special sympathy with wolves.