beat makin question (beg.)

quick question, i am a beginner so bear with me. say i made a beat usin the pads and they are all on track 1-4 (kick on tr. 1, sn. on tr2. etc) okay.. now say i came up with this nice riff on the motif... what is the best/most efficient way to integrate the riff into the beat? like if i wanted to lay it on track 5?? i know i could just sample it.. but i wouldnt be able to hear the beat at the same time.. so, what if my timing is off when i play it?? basically.. is there anyway.. to play the beat i already made.. and then sample the riff i am playing on the motif to the mpc.. this way the timing would be in sync with my beat??? see i would rather build a beat first, then use that to mess around with the motif to come up with a riff.. but my problem is trying to bring that riff into the mpc while staying on time... with the beat.. any help would be appreciated.. thanks for ure time

what I wuld do on gthe krokus mane is use that there midi.....or if you have recorder just record your riff into the 2nd channel while the mpc beat is playing and then sample that into the mpc....or you can use your mpc to trigger the riff from the keyboard via midi...