Difference in redshift between 2 interacting galaxies
From eq. 10 in Hogg's classic paper, assuming that the peculiar velocity $v_\mathrm{pec} \ll c$:
$$v_\mathrm{pec} = c \frac{z_\mathrm{obs} - z_\mathrm{cos}}{1 + z_\mathrm{cos}},$$
where $z_\mathrm{obs}$ is the observed redshift, and $z_\mathrm{cos}$ is the redshift from cosmological expansion only.

Let me invert that and wrap it up in Python for ya:

def zobs(vpec,zcos):
  c = 3e5
  return (1+zcos) * vpec/c + zcos