If the QT has ever had medications in it, then no it is not a good idea. Definately NO, if the QT ever had copper in it.
If the QT might have fish diseases in it, then no it is not a good ideal.
If the QT has only been used for hyposalinity or housing new fish, then it would be OK. Better to just get a rubbermaid for the LR and throw in a couple of PHs.