I'm new to Mac and its software. But when I had my PC, I used a plug-in from Native Instruments with Fruity-Loops. So I'm wondering: Can I do the same with GarageBand?
I believe yes, with limitations. The last time I used Garageband was version 2, and at that time it could host Audio Units (but not VST) plugins. I presume it's the same now.
I don't know which Native Instruments plugins you're thinking of (Native Instruments is a company, not a product) but if any of them are offered in Audio Units format, Garageband should be able to host them.
It can indeed be done. If you have installed the Native Instruments plug-in on your system, you can create a new software track in Garageband, click the little "i" icon in the bottom left corner, chose "edit" in the menu that pops up and then in the drop down box under "Sound Generator" chose you plug-in. All set!