Hello!

Yes, there is!
What you need to do is:
Connect a couple of outputs to your Lexicon Inputs(I suppose it's a stereo effect unit), let's say Output 3 and 4
Then connect the Lexicon's Outputs to a couple of Inputs, let's say Inputs 3 and 4 in the FF800.

Now in order to apply the Lexicon's effect to where you need it:

1. Select your Main Output(third row in TotalMix FX) by clicking on it
2. Raise the faders corresponding to where you have the Lexicon's Outputs connected to(An3/4 in this example), to "0", in the Inputs section of TotalMix FX(First row)
3. Now, in the third row, select the Outputs where you have the Lexicon Inputs connected to(An/3/4 in this example)
4. With these outputs selected, go to the first row and raise the faders of the Inputs you have your synths connected and you want the Lexicon to be applied to!