Download your mods of choice to someplace you can find them (i.e. your desktop, or a special temporary folder).
Open your FTB Launcher. Select the pack you want to edit (mine, in this case) and click edit modpack.
I don't remember exactly beyond here, but I think you'll want to look for a button that says "add mods."
Select the mod or mods that you want to add. One-at-a-time works for sure, but I haven't tested whether you can add mods in bulk.
Alternatively, you can find the directory of FTB (which is shown in the options tab of the launcher), find my pack's directory, and just drop your mods in the mods folder of that directory.