Please retain the user list in your replies. The on-line documentation describes all the commands you need to do what you want.
Jeremy
I'm sure there are lammps input script commands which can do what you want.
However if I were in your shoes, I think the easiest thing to do is
just to add some additional lines to (the end of) your "Atoms" section
of your data file (one line for each new atom).
You can also bond these atoms to the other atoms in your simulation by
adding lines to the end of the "Bonds" section. If there are
angle-dependent interactions involving these atoms, then you will have
to add lines to your "Angle", "Dihedral", and "Improper" sections as
well. All of this can be done by modifying different sections of your
DATA file (manually).
So, if you are not yet familiar with it, I suggest that you get
familiar with the DATA file format.
The official documentation for the DATA file format is here:
http://lammps.sandia.gov/doc/read_data.html
In my opinion, the easiest way to learn about data files is to look at
an example. I like the example located in "peptide" directory in the
"examples" directory (included with LAMMPS).
(I think most users either write their own DATA files manually, or use
programs to build them which they write themselves or find on the
internet. <<obligatory self-promotion>> If you haven't yet, you
might also take a look at http://moltemplate.org. Moltemplate is a
general LAMMPS data file builder (designed for simulating finite-size
molecules). It can be set up to generate the angle,dihedral,improper
interactions automatically.)
Cheers
Andrew