Passing channel to Avalon-ST demultiplexer

i don't completely understand what your system looks like, but it sounds like you may want to drive the channel input yourself? SOPC Builder wants to wire the channel I/O from one ST block to the next. if both blocks do not have the channel signal you'll have to look at making your own demux block with the select line defined as a conduit, or at channel adapters (though i'm not sure how the select line would work in this case)