I ve got the 555 only yesterday
Usually I play some pad/pads and add to loop capture internal sounds (pad performance or d-beam synth). These new sounds adds to playing pads sounds to loop captured sample.
But it is possible to record new performance to loop capture separately.
1) for pads that will play - mfx b9 and pan to right (or left)
2) connect one (!) cable from left (or right) main out to mic hi-z input. You need to choose 1 output where is not presented sound of playing pad/pads
3) source select - ext only
4) now loop captured sound records separately of played pads.
So I have playing pads as rithm/base track and record pad performance on separate track (loop capture -> save to pad)
Hope it s useful info