ConTeXt: How to append content to already used buffer

When you label a buffer, it is already accessible to getbuffer[label]. definebuffer has other purposes. To answer your questions I suggest two ways:

A nice approach from the mailing list

The following example does a better handling of buffers.append. I've just adapted it from the mailing list. It defines startappendbuffer and stopappendbuffer so it's more comfortable to use:

startluacode
    userdata = userdata or { }
    function userdata.addTempToBuffer(buffername)
        buffers.append(
            buffername,
            'n' .. buffers.getcontent('APPENDTEMP')
        )
    end
stopluacode

defstartappendbuffer[#1]%
{defstopappendbuffer%
      {ctxlua{userdata.addTempToBuffer('#1')}}
   dostartbuffer[APPENDTEMP][startappendbuffer][stopappendbuffer]}

starttext

startbuffer[x]
Hola
stopbuffer

startlines
getbuffer[x]
stoplines

startappendbuffer[x]
input knuth
stopappendbuffer

startlines
getbuffer[x]
stoplines

stoptext

A naïve approach: wrap buffers.append

There's already a Lua function to append content to a buffer, i.e. buffers.append(buffer, text) (see buffers in LuaTeX). So we're going to wrap it using ctxlua, which serves to call Lua functions in TeX:

%#1 is the name of the buffer
%#2 is the content to append
unprotect
defappendtobuffer#1#2{ctxlua{buffers.append("#1", !!bs#2!!es)}}
protect
starttext 

startbuffer[lol]
dorecurse{6}{hello! }
stopbuffer
getbuffer[lol]
appendtobuffer{lol}{startformula e^{ipi}+1=0stopformula input knuth}
getbuffer[lol]
stoptext

Notice the use of !!bs and !!es so your content is passed as a string to Lua. I'd use it if I want to add simpler, shorter snippets of text because, as Henri Menke points out (thanks! ?), it doesn't handle line breaks and expands its content, which is troublesome in special cases, among other unexpected effects.