There's always the possibility that the queenless colony (especially if it's been queenless for a long time), has developed a problem with laying workers. If that's the case, then it would possibly be ill advised to attempt to directly combine the package with the queenless colony. I can think of several ways that might reduce the risk. If when you have the package, and it's time to install it, you then carefully examine the queenless colony, to ensure laying workers have not begun filling cells with worker-produced eggs. You could also place the package queen, in her cage, with the queenless bees, and observe how they respond to her. If they respond with excitement, fanning, and moving in coordinated patterns or do they immediately begin attacking the cage and trying to kill the bees in the cage, including the queen. Attacking bees will cling to the cage so fiercely, that it is difficult to dislodge them from it. Accepting bees, though they show great interest in the caged queen, can easily be pushed around while they're on the cage, though it may be difficult to remove them.
Other possibilities, include installing the package, adjacent to the queenless bees, separated by a double screen board, either above or below them, giving them separate entrances. Then, once the package bees have combs with eggs, swap a frame of eggs, with an empty from the queenless bees. Once the frame of eggs is capped brood, they should be ready to remove the double-screen and combine them.