You can certainly add Dichlor to your pool by just throwing it in, but just keep in mind that for every 1 ppm FC that you add (i.e. the amount that you throw in that produces 1 ppm FC), you also add 0.9 ppm (almost 1 ppm) of CYA. So if you are low in CYA, then there's no problem; otherwise, I wouldn't use the Dichlor. I suspect that since you are using Tri-Chlor in an automatic feeder that your CYA level might be high already. Have you tested it? Unless your pool is small-ish (10,000 gallons or smaller) and do frequent backwashing, your CYA is probably already too high due to your use of Tri-Chlor.

Richard