How to import Markers via OPML?

I've determined that the following attributes work in OPML to import into Xmind:

1. _url for weblinks
2. _note for notes
3. _label for labels (comma separated)

However, I've not been able to determine how to import Markers. I've tried:

_markers="tag-yellow"

and 

_marker="tag-yellow"

But neither work. Could you provide instructions on how to import Markers via OPML?