Technic does not support that (custom ZIPs) any longer on the new Platform. However, you can still install extra mods onto the modpacks like it is your normal .minecraft folder. Go to your AppData\Roaming\.technic (or whatever folder chosen for the TechnicLauncher files). Then look for the modpack you want to download to. Lets say it's Voltz( can be any modpack). Look for the "voltz" (or tekkit/technic/hackslashmine or anything, depending on the modpack you want to download the mod to). Open it and you will see something that looks like your .minecraft folder. Depending on the mod you want to install, either go to the "bin" folder and copy the files into the minecraft.jar archive or put the mod files into the "mods" folder. (Read instructions on the modder's page. I will not be held responsible if your Technic crashes for any reason.)