in reason 5 you would need to create a combinator with a Thor module in it
you use the audio ins on the Thor to route via the matrix to a low pass filter, and connect the outputs of the filter back out to the audio outs of the Thor and then to the combinators outputs
attach the frequency cutoff of the low pass filter to one of the combinator knobs
send audio to and from this combinator as you need
then automate the combinator knob for cutoff freq as needs be - right click -> edit automation etc....