Just be aware that that ain't as easy as opening a graphic, selecting something, changing the opacity, saving and voilá.
Some files can be in svg format, some can be in svgz format. svgz is just a gzipped svg but I'm not sure if Inkscape can open them directly.
Then most probably the svg elements that build the parts of the panel or the plasmoid can and most probably would be grouped. So you need to ungroup them first and be aware of that. Maybe they're grouped because that's how Plasma seeks them as an individual element and, if it's that, they will have an special id associated for them so Plasma can find them (iirc in Inkscape you can check for that in Object->Properties or something like that, a popup dialog will appear). So take note of those ids.
Once you're done editing your stuff be sure all elements are grouped and id-ed as they came originally and then you can save the file(s) so Plasma will find the files and elements correctly without messing them up.
(See, that's why I hate that Plasma relies on SVG for all of that stuff. The very fact that you require a non-kde app to create/edit/update them is unbelievably stupid)