Putting in a heteroatom

I have modeled a large protein (1058 residues) based on the Xray structure of a homologous protein. The homologous protein (1EUL) has two Calcium ions, while the homologue I have modeled should have one Calcium ion, in the position corresponding to one of the Calciums in 1EUL. I can't figure out how to introduce this Calcium ion into my model. Can anyone advise me? Thanks, John