foo = {}
foo[#foo+1]="bar"
foo[#foo+1]="baz"
This works because the # operator computes the length of the list. The empty list has length 0, etc.
If you're using Lua 5.3+, then you can do almost exactly what you wanted:
foo = {}
setmetatable(foo, { __shl = function (t,v) t[#t+1]=v end })
_= foo << "bar"
_= foo << "baz"
Expressions are not statements in Lua and they need to be used somehow.