There is an experimental version of Inkscape with mesh gradients which would let you acheive what you want, but (a) it's experimental and (b) the syntax for mesh gradients hasn't yet been ratified by the SVG working group (it should be in SVG 2.0 in a year or two), so future versions of Inkscape may not read files created with it right now.
Alternatively your best bet is probably to use blurred highlights/shadows with clipping to simulate the gradient you want. See my latest Inkscape tutorial in Full Circle Magazine (it's a free download,
http://fullcirclemagazine.org/issue-73/ ) which is all about clipping, and shows a simple example of simulating a conical gradient in this way.