That's an easy one - any 'blue', that is magical, item can have it's mods re-spawned in random fashion by tossing it in your horadric cube along with three perfect gems and pressing the transmute button. If you wish your Grand Charm to spawn a + to skills mod, you'll need to use a charm with a high item level - I believe the Grand Charms found in the Great Marsh (Act 3) in Nightmare are of sufficient level to possibly spawn skills. The highest level Grand Charms, those dropped personally by Diablo and Baal in Hell, have the ability to spawn the very best mods, the most prized being + to skill and + to life (one prefix, one suffix).
Even with a high level charm, re-rolling a charm into a 'skiller' is an uncommon event: I'm told the average number of re-rolls is 22, and can run much higher than that.
Elgar, who posts on this forum, is an avid charm-roller. He can help you out more than I am able to.