If it's just a purely temporary fix for the cold feed for the washing machine then yes you can use the filling loop feed pipe IMO, disconnect the fill loop bottom iso valve nut off and screw in a WM valve Tee and then re-attach the fill loop valve above with a little piece of pipe.
In fact the fill loop flexi should be disconnected anyway in normal use
In fact the fill loop flexi should be disconnected anyway in normal use