Should we bring another cat into the home?

A family friend's cat had kittens several weeks ago, and they're outdoor cats. Currently, we have a "senior" dog and a two year old cat (who came from the same household as the kittens, and is very spoiled). The cat was an...unexpected family addition, but we've enjoyed having his smart-but-grumpy personality, and the friendship that he and the mellow "princess" dog have is adorable. Lately, he's been playing less and less, and he will just meow loudly for no reason. We umed that he was sick, but he went right back to his former spunkiness when he saw cats on television. Now, we think it could be lonliness. Should we bring another cat/kitten into the home, since the barn cats would probably die anyway? Would that unnerve the cat, or make the dog feel neglected? Would there be a lot more work than we currently have? Please answer, and thank you!