webgis, projections, josm.

Recenctly I got access to a webgis that I’d like to have it added as a (pseudo)wms layer to JOSM.
I’m fairly new to josm, hence I don’t know if this is completely feasable and to complicate things further the webgis is using an ESPG:3003 projection. How may I force JOSM to make queries with that projection?
I tried to use the proj4 plugin, but didn’t find a way to get it correctly configured (documentation seems to be somewhat lacking)…
Any advice? thx!

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