Clearly - pharases "in relation to which" (subordinating conjunction) function as one word. How such process is named in linguistics. It would also be interesting to know how such formation is handled in Grammatical Framework and Abstract Categorial Grammars generally? Is it allowed to handle such phrases as one lexical unit?
The term for such kind of phrase is multiword expression. I am not aware of a special term for the process that creates multiword expression. I am also not aware of some special treatment of them; in corpus linguistics usually each word is tagged separately with a part-of-speech tag and syntax trees are built from the single words (as separated by spaces).