motif: A string with what motif should be added to the Pose. For example, "10HA" adds 10 helical residues. "1LG-1LB-10HA" should be valid but this author doesn't know what that does.
chain: An integer, with which chain of the Pose should be modified. Notice this is number not letter; you'll need to know the internal numbering of the Pose.
prepend: by default, append to the C-terminus of the chain; if prepend is true, it probably goes to the N-terminus instead
segment: a different way to specify what to modify, other than by chain number; this author doesn't know how it works.
length: NOT a valid option! You have to use motif instead.