The game automatically welds words it detects are valid. However, proper nouns and some more archaic words without definitions are not available to be welded. Try going to the Menu (green button) then lookup the word you are trying to spell in the word lookup box to see if it is available in the game.
If you have anymore questions about the W.E.L.D.E.R. word list you can follow the link below that explains the list in greater depth.