If you are using the Curse Launcher I can't help you.
If you are using the FTB launcher to download a pack:
Launch FTB and run the modpack you want to use to download it / update it to the version you have selected. Once the client launches exit.
If FTB was "installed" into C:\FTB, and you had downloaded "Agrarian Skies" there will be a folder of interest:
c:\FTB\AgrarianSkiesHQ\minecraft
This folder will contain config, mods, custom assets, scripts and other stuff.
Now, open your Vanilla launcher. Create a new profile and call it something. e.g. "AgrarianSkies"
Make sure the profile is pointing at the correct version of Forge / MC for the modpack.
Set a custom profile path. "C:\MinecraftProfiles\AgrarianSkies".
Make sure the custom Java CLI is enables so you can edit / add a -Xmx parameter with at least 2048M.
Copy the contents of C:\FTB\AgrarianSkiesHQ\minecraft, into C:\MinecraftProfiles\AgrarianSkies.
Launch the profile and it should load.
Using MultiMC there is an import option that I have not gotten to work. But the process is the same. Create an instance in MultiMC, and ensure it is the correct version of Minecraft for the modpack. Then edit the instance and click the "Add Forge" button and choose the appropriate version of forge. Open the instance folder - if you had installed MultiMC to C:\MultiMC, the folder will be: C:\MultiMC\instances\AgrarianSkies\minecraft (iirc), and copy in the contents of C:\FTB\AgrarianSkiesHQ\minecraft into that.
Edit the instance memory allocation to be larger and launch it.