Currently, for MOGRTs with placeholder media, you can only add clips from the Project panel.
These clips then default to 00:00:00:00 timecode within the template. The current workflow is to either manually seek to the start time you want, create subclips prior to bringing into the template, or pre-render new clips altogether, with those times baked in.
A better solution would be to have the ability to just swap template placeholders for clips already in the timeline. Remove the added step of subclipping or timecode seeking altogether. That all happens automatically. Ideally, there'd be no new clip or projectItem created at all. IE, no extra clutter in the Project Panel.