Certainly. You can use the following anywhere outside a language file, as long as it's on the client (ie. use "if CLIENT then" where appropriate):
LANG.AddToLanguage(lang_name, string_name, string_text)
So like:
LANG.AddToLanguage("english", "my_text", "Here is some text")
One use for this is adding a proper name for ammo types that are not used in default TTT. Say you have a weapon using the "XBowBolt" ammo type (perhaps with a separate new custom ammo entity) so it doesn't share ammo with the default guns, and you want it to say "Wooden Sticks" when you pick up some of that ammo. You could add a language item by doing:
LANG.AddToLanguage("english", "ammo_xbowbolt", "Wooden Sticks")
When picking up ammo TTT looks for a language key called "ammo_<ammo type>", so it will find your definition and use the custom text in the little popup on the right side of the screen.