If you were building out from the C-term, the "addaa" command would probably be the easiest approach in Chimera.
<http://www.cgl.ucsf.edu/chimera/docs/UsersGuide/midas/addaa.html>

However, for putting residues on the N-term or for joining two experimentally determined structures, Join Models (part of Build Structure) is generally the way to go.  Start Structure (another part of Build Structure) allows creating peptides with specified phi,psi angles.
<http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/editing/editing.html#build>