Simply put: No the current launcher can not do that right now. It would need further adjustments to allow Retro-Modpacks.Slowpoke and/or anyone else from the modpack team: Is the update to 1.6 going to also allow for any and all versions of minecraft? Up until now, FTB has used the legacy update system for minecraft versions, and 1.6 is not hosted on these anymore, it is hosted on a system which contains all versions of the game so people can select any one with the launcher.
So my question is: Are you going to be adding support to use pre-1.2.5 versions? I was going to make a Beta 1.7.3 modpack, but FTB did not support Beta versions at the time.