You cannot do it at install time (for now, I hope we will improve that soon).
What you should do is to compile your LESS into CSS by yourself in your add-on sources (2 solutions: either you already have a prefered way to compile less, either you just build your bundle using the Plone resources registry on a local instance and you download the resulting css), and declare this compiled version in your registry.xml, like that: https://github.com/plomino/Plomino/blob/plone5/src/Products/CMFPlomino/profiles/default/registry.xml#L123 (here it is a js bundle, but that would be the same for a css one).
Currently you cannot compile your bundles while installing. You have to provide it precompiled and live with an extra request (only bunldes are loaded in a rendered site). This is also the reason, why you cannot add it to an existing bundle, like the "plone" bundle - you cannot precompile the plone bundle because you simply don't know what the "plone" bundle contains. There might be solutions, like automatically compiling all bundles at install time TTW, which are modified by an addon.
For CSS only, you can add your CSS resource to the "plone-legacy" bundle. All resources in there are just concatenated. There is no need to compile it.
I have not yet had a close look at Plone 5. However, I am quite confident that Plone 5 has retained "GenericSetup" as important configuration tool. In this case, you could use a specialized "GenericSetup" import step for your add-on to do all the necessary things which cannot be done in other ways.
Please correct me if I am doing something wrong, but I might have noticed some inconsistencies in the way new resource registries include the bundles in final output. It seems that when Diazo is enabled, resource registry invokes my theme's resources twice (both CSS and JS).