How would I go about adding somthing later? And what types of stuff would be best suited for the belgian wit?
Taste the beer when you take your FG reading. If it doesn't taste right, you can add more fermentables, hops or spices to compensate, using the proper formulas or brewing software to guide your additions. For instance, if your beer tastes way too watery and you don't think carbonation alone will be sufficient to make it enjoyable, you'd do a short, very concentrated wort boil, chill it as usual, and add the new wort to the fermenter. I've salvaged a couple of subpar batches that way, and they turned out nicely.
Let common sense be your guide: If you need more bitterness, boil some hops in dilute wort. If you need more aroma, consider dry hopping. If you need more body, consider adding some amber or munich malt extract, etc.
For a witbier, your options are pretty much wheat extract, maybe some hops and spices, like orange zest, chamomile, juniper berries, etc.