Torey Krug will be a RFA this summer. If the Wings want to pursue him then they would have to forward him an offer sheet. If Krug accepted the Wings' offer sheet then Boston would have a chance to match that offer. If the Bruins match then he would stay in Boston. If they did not match Detroit's offer then he would become a Wing, but the Wings would owe the Bruins some draft picks - the exact quality of those picks would be determined by Krug's salary.
In short, yes... it's possible to land Krug, but unlikely since Boston would likely match any offer by the Wings - unless there was overpayment. RFA can be poached, but they can only be poached if you're willing to overpay. If Holland offered Krug 4-5 million then I'm sure he would be able to get him; any less and I think the Bruins match.