All extra POIs are stored in a data file called
poi.gpi
which must be placed in the folder /Garmin/Poi/
A file like this doesn't use any Waypoint memory.
They can then be found in the menus under 'Where To' and a new heading 'Extras'. I have this working on a E30x.
If you have several datasets from various sources (eg pubs, bike shops, funeral parlours), they need to be combined into the one file.
Possibly you can have one file on the GPS and another on the mSD card, I haven't tried this.
I think there are free tools downloadable from the Garmin website, to construct a poi.gpi file from GPX.