Okay, I see what you mean.
Yes, you do have a pack+head system. In studio lighting terminology, a "pack and head" is exactly as you describe. Monolights are totally self-contained units that each plug into 120V(or 240V) and have their own controls built in including the trigger. Both have their place, but I'm definitely a pack+head guy even if I'm out of the studio.
I'm not a big fan of optical slaves as a whole, but even the pop-up SHOULD have enough power to trigger them. Use some velcro or whatever you have to make sure the "bubble" of the slave is facing your camera.
As best as I can tell, the set I linked CAN be used just as a generic("dumb") radio trigger although it could also be used to do off-camera TTL with Canon. Poke around and they may have a Nikon version, but honestly I'd not mess with it. Use CLS if you want to use Nikon Speedlights off-camera.
If you want to be "fancy" and do it right, you can add a PC sync socket to your camera with a Nikon AS-15. It fits in the hotshoe and is $20. There's nothing special about it(I don't think) and if I didn't have one(got it with some other stuff)...plus the fact that every Nikon SLR/DSLR I use has a PC socket...I'd get a generic one for half the price. There's nothing exotic about it.
I will add two cautions, though:
1. GOOD PC sync cords are expensive, and cheap sync cords can cause more grief than they are worth. Before I had radio triggers(which came with my Norman kit when I bought it) I did a lot of off camera work with corded connections. Even the best cords have a nasty habit of coming loose at the most inopportune times, while also sticking like glue of someone trips over your cord and instead knocks your camera or lighting equipment over. Your pack probably has what's called a household plug on it for sync-pre-made household-PC cords tend to be fairly high quality. You can also make your own long cords from hardware store parts and only use a pre-made cord at the very end, but I consider this dangerous lest some well-intentioned person inadvertently plugs one into an outlet. And, again, EVEN THE BEST PC CORDS FAIL AT THE WORST TIMES.
2. Many older packs have little regard for sync voltage. It was expect that they'd probably be used with cameras that have mechanical sync contacts, and in that context it doesn't matter. You can have sync voltages of several hundred volts. Nikon officially says that a built-in PC socket on a modern camera is good up to 250V, but is a bit mum on what's safe through the hot shoe. I don't lose too any sleep over the ~8V sync voltage on my Quantum triggers, but a good GENERAL rule is that you should probably keep the hot shoe voltage to 6V or lower. There are products like the Wein "Safe Sync" that address this, but you'll pay just as much for radio triggers as you will for one of these plus good cords.
All I can offer in addition is to have fun-studio lighting is so incredibly versatile and opens up a huge world of possibilities. My Norman stuff is old enough that many folks have moved onto more advanced stuff. As a result, I've ended up with 16 heads, 5 power packs(in power ranges from 800 w-s to 2000 w-s), and virtually every modifier that Norman ever cataloged. My initial purchase was from the estate of a local pro, but I've since had stuff given to me or sold at give-away prices.
I will add that if you don't have any modifiers, I'd suggest picking up just some basic ones to get you started. Depending on what you do, you'll probably eventually want to get some soft boxes(usually bigger is better) and also some "hard" modifiers like grids. For starters, though, a couple of good umbrellas are a nice and cheap starting point. B&H has some nice quality 36" umbrellas for around $10 each(new). As a quick education, they come in either reflective or shoot through varieties, and each has its place. Reflective are more efficient but also a bit "harder". Shoot through are a lot more diffuse. For starters, I'd suggest two silver(reflective), two white translucent(shoot through) and perhaps one or two white-on-black(reflective). That will get you started. Since you have 3 strobes, a 12-24" beauty dish(white is a bit "tamer" for starting out, while silver is more efficient and "harder") is a cheap and versatile addition.
