No there isn't. For configuring sampling activity, the DAPL system must build some complicated control structures to represent your channels, their gains, their signal connections, storage buffers, synchronization switches and so forth. Whether the configuration is feasible can depend on the number of channels. Whether the channels are present or not determine the timing requirements. So the DAPL system really has to know everything about the sampling configuration, and then it is committed. It is too late to change this by adjusting some kind of macro or parameter.

Here is an alternative that does almost the same thing. Your DAPL configuration allows you to define multiple sampling configurations. You only use one of them at any given time.

    constant  NCHAN word = 0

    idefine  ONECHAN  
      channels 1
      set ip0  S0
      time 1000
    end

    idefine  TWOCHAN
      channels 2
      set ip0  S0
      set ip1  S1
      time 500
    end
  
    idefine  THREECHAN
      channels 3
      set ip0  S0
      set ip1  S1
      set ip2  S2
      time     333.32
    end
  

And now you can pick which sampling configuration to run by sending two command lines instead of the usual one command to start the processing.

    let   NCHAN = 2
    start TWOCHAN