Can i switch from vanilla to modpack?

so i want to complete vanilla minecraft and then add mods but idk if im going to lose all my progress.
lets say we kill the ender dragon right and then we want to add the twilight forest mod will it crach the server or should i do it from the start.

if this was discussed already can you send me the link ?

AND !! i got 2 friend that dont have mc so they will join with cracked , and if i have a modpack can they join ?