It's very easy to do. Just click on the IP Object that you want to modify->Click "New"->type in the IP Address->Click the drop down for Type and select the type (most likely Device Tags, at least for me)->Click Apply. Repeat this for each IP that you need to add to the object.
If you have a lot of IPs to add to the object, then you can import a csv as Eddie mentioned. The format is a little confusing, so you may have to do a few test runs.