easy as pie, everything that goes to the dac~ adds together (sfplay plus your adc~ input, plus anything else). Try multiplying / convoluting the signals before they go out, can be interesting. Also an LFO is cake, just *~ a signal with a low-freq cycle or other modulator wave. (or a high-freq one, etc. etc.)
For effects I love the filtergraph~ / biquad~. lots of cool possibilities there. just watch your levels, they can get real loud real fast.