Depends on the "custom data". You will need to convert it to one of the organized types in XData (check the AutoCAD DXF reference for code ranges and applicable types). For example, if your custom type contains a couple of INTs, a boolean, and a string then its pretty simple to chain it together. Most types are supported in one format or another, plus a few AutoCAD-centric types like object references. I *think* there's a section in there for binary storage, but watch out for the XData size limits.