Do you mean having Loop20 processed by a chorus effect?
If that is the question, you simply need to create a preset which has 2 fx blocks, with LOOP 20 in A and ST.CHORUS in B, with a SERIES routing.
Load LOOP 20
press PARAMETERS twice & press ROUTING
press PARAMETERS again then press PROGRAM as first softkey in the display
use the knob to reach 115 St.Chorus, then press LOAD FX B to load the chorus in the B block.