If you wanted to route an external audio signal into the effects processor in the K2000, you have to use an insert cable (or cables) plugged into the A separate outs.
An insert cable is a has two mono jacks Yd to a single stereo jack. One cable is the send and the oyther the return. You would plug the output of your external source into the return side of the cable and plug the stereo jack into the A seperate out. The signal would then go through the effects processor and come out of the mix outs.
The problem with this method is that by plugging a cable into the jack, you are removing the K2000 signal routed to that jack (it is being sent out of the send side of the insert cable. If you wanted, you could pan all K2000 signals to one side (for example A Left) and then use A right for the external signal. Then in the Mix outs you would have the dry portion of the signal coming out of its respective side, while the wet portion of both K2000 and external signal comes out equally on both MIX outs.
If you want to process an external signal AND use both left and right sides for the K2000 signal, you need an external Mixer. You would plug the two (L&R) sends from the insert cables into two channels on the Mixing board, put your external signal into another channel, then use a submix to send the returns back into the K2000.